Fabrication process for a/m/x materials

ABSTRACT

The invention relates to a process for producing a crystalline A/M/X material, which crystalline A/M/X material comprises a compound of formula [A]a[M]b[X]c wherein: [A] comprises one or more A cations; [M] comprises one or more M cations which are metal or metalloid cations; [X] comprises one or more X anions; a is a number from 1 to 6; b is a number from 1 to 6; and c is a number from 1 to 18. The process is capable of producing crystalline A/M/X materials while precisely controlling their stoichiometry, leading to products with finely tunable optical properties such as peak emission wavelength. The invention also relates to process for producing a thin film comprising the crystalline A/M/X material of the invention, and to a thin film obtainable by the process of the invention. An optoelectronic device comprising the thin film is also provided.

The project leading to this application has received funding from theEuropean Union’s Horizon 2020 research and innovation programme underthe Marie Sklodowska-Curie grant agreement no: 706552.

FIELD OF THE INVENTION

The invention provides a process for producing a crystalline A/M/Xmaterial comprising a compound of formula [A]_(a)[M]_(b)[X]_(c), where ais a number from 1 to 6, b is a number from 1 to 6, and c is a numberfrom 1 to 18. In particular, the invention provides a process forproducing a thin film of said crystalline compound. Also provided are athin film obtainable or obtained by the process of the invention, and anoptoelectronic device (such as an LED or solar cell) comprising saidthin film.

BACKGROUND TO THE INVENTION

There is great demand worldwide for photoactive semiconductor materialswhich convert electrical energy into light and vice versa. Suchmaterials are useful in a wide range of semiconductor devices. Forinstance, light-emitting materials are useful in applications such asthe production of luminescent screens and light-emitting diodes (LEDs).Light-absorbing materials are in high demand for the production of solarcells.

Organic species are currently widely used in fabricating light-emittingmaterials. However, organic species suffer from the disadvantage thatthey often emit weakly in the blue region of the visible spectrum. Forexample, organic LEDs (OLEDs) typically have a conversion efficiency ofup to 20% in the red and green regions of the visible emission spectrum.However, they can often only achieve conversion efficiencies of up to10% in the blue region of the visible spectrum. One key reason for thisis that efficient red and green emission in OLEDs relies on theradiative recombination of all single and triplet excitons.Triplet-triplet electron transitions, which are generally spin-forbiddenin organic emitters, is made possible by the introduction ofphosphorescent metal complexes. However, such metal complexes are notavailable or lack stability and efficiency in the blue region of thespectrum.

Lead halide perovskites are desirable candidates as photoactivematerials as they are highly emissive semiconductor materials. Highlyemitting perovskites is taken to mean perovskites with a radiativeexternal quantum efficiency of 20% or more in a thin film. Lead halideperovskites are materials of formula APbX₃, wherein X is a halide.

Metal halide perovskites and particularly lead halide perovskites areparticularly promising semiconductor materials as it has been shown thatthe band gap of such perovskites can be tuned by adjusting theirchemical composition. For instance, a study carried out by Linaburg etal (“Cs_(1-x)Rb_(x)PbCl₃ and Cs_(1-x)Rb_(x)PbBr₃ solid solutions:understanding octahedral tilting in lead halide perovskites”, Chem.Mater., vol. 29, pp3507-3514, 2017) shows that varying Rb content in aceasium lead halide perovskite can change the band gap significantly. Atunable band gap is advantageous for semiconductor materials used inoptoelectronic applications as it allows precise engineering of thewavelength at which the material absorbs or emits light.

In order to precisely control the band gap of the perovskite, it isimportant to obtain high quality perovskite materials in the solid stateand to accurately control their stoichiometry. The known processes forproducing perovskite materials leave room for improvement in thisregard. A typical wet process used to synthesise thin films is describedby Poglitsch and Weber in “Dynamic disorder in methylammoniumtrihalogenplumbates (II) observed by millimetre-wave spectroscopy”, J.Chem. Phys., Vol. 87(11), pp6373-6378, 1987). Separate chemicalprecursors (in this instance CH₃NH₂ and PbX₂ wherein X is a halide) aredissolved in a concentrated aqueous solution of the hydrohalic acid HX.The solution is then cooled, causing crystals to grow due tosupersaturation of the solute occurring as the temperature drops.

A particular difficulty with these wet processes is that the solubilityof the various chemical precursors in the crystal-forming solution canbe very dissimilar. Difficulties are therefore encountered in fixing theconcentration of each precursor in the crystal-forming solution at thedesired value, particularly if the precursors’ solubility varies withtemperature. Often, the optimal composition of a solvent for thepurposes of dissolving the precursors will differ from the optimalcomposition of a solvent for crystallisation (by cooling, spin-coatingor other crystallisation methods). Compromises must often therefore bemade, necessitating the choice of a solvent with less than optimalproperties for the purpose of dissolving the precursors and forpromoting crystal growth.

It is therefore very difficult to achieve precise stoichiometric controlusing these known wet processes. Imperfect control of stoichiometryleads to crystalline products and thin films with a poor definition ofthe band. It renders controlling the band gap through precisecompositional control highly difficult. In addition, poor control ofstoichiometry can lead to crystals with a high density of crystaldefects, such as vacancies, interstitials and anti-sites, which can leadto a high density of electronic defects in the material.

Moreover, compromise on the temperature or solvent used duringcrystallisation in order to improve solubility of the precursors canlead to sub-optimal crystallisation conditions and reduced quality ofthe crystals and thin films thus obtained. Poorer-quality crystals andthin films may contain more defects, leading to poorer optoelectronicproperties, and may be more susceptible to degradation.

The problem of differing solubilities of precursors has previously beenaddressed using vapour phase deposition techniques. However, theseprocesses are inconvenient as they are slow and require complexequipment.

Consequently, there remains a need for a process which is simple toperform but which can provide crystalline products and particularly thinfilms of high quality and with precisely-controlled stoichiometry, andhence with a precisely tunable band gap.

SUMMARY OF THE INVENTION

In general, the salts used as precursors in the fabrication ofcrystalline A/M/X materials have different solubilities. For example,the solubility limit of inorganic halide salts (e.g. halide salts of Cs⁺and Rb⁺) is generally lower than the solubility of halide salts oforganic cations (e.g. halide salts of CH₃NH₃ ⁺ and CH(NH₂)₂ ⁺) inorganic polar solvents. These different solubilities complicate thesynthetic route to crystalline A/M/X species, especially where two ormore different cations are required in the crystalline A/M/X product.Moreover, the optimal solvent composition for the dissolution of the ofthe precursors is often different than for the crystallisation duringthe spin-coating process. As a result of these difficulties, it isgenerally difficult to precisely control the stoichiometry of thecrystalline A/M/X product.

To overcome these problems, the inventors have provided a new syntheticroute which includes treating an aqueous solution comprising one or moreA cation precursors with an M cation precursor in an organic polarsolvent, which organic solvent typically also contains a concentratedhydrohalic acid. On such treatment, the stoichiometric crystalline A/M/Xmaterial precipitates. The invention therefore provides a process forproducing a crystalline A/M/X material, which crystalline A/M/X materialcomprises a compound of formula:

-   wherein:    -   [A] comprises one or more A cations;    -   [M] comprises one or more M cations which are metal or metalloid        cations;    -   [X] comprises one or more X anions;    -   a is a number from 1 to 6;    -   b is a number from 1 to 6; and    -   c is a number from 1 to 18,-   the process comprising:    -   a) contacting an aqueous solution comprising an A precursor and        an aqueous solvent with an organic solution comprising an M        precursor and an organic solvent; and    -   b) allowing a precipitate to form when the said aqueous and        organic solutions are contacted.

The above method advantageously allows excellent control of thestoichiometry of the [A]_(a)[M]_(b)[X]_(c) product. The above method isassociated with surprisingly lower instance of side-reactions, whichform unwanted A/M/X side-products and affect the molar ratio of the A, Mand X species during the process. In consequence, the above methodyields crystalline A/M/X materials which have reduced contamination withundesired A/M/X side-products compared to the methods of the prior art.Thus, the process of the invention produces a product with high purityand good yield having precisely-controlled stoichiometry.

Further, it has been surprisingly found that, by taking the product ofthe above process and using that product as a starting material to forma thin film, the thin film thus produced has very finely tunable opticalproperties.

Thus the process is typically a process for preparing a thin film ofsaid crystalline A/M/X material, and the process typically furthercomprises: (c) optionally washing the precipitate; (d) vapourising theprecipitate, or dissolving the precipitate in an organic solvent to forma film-forming solution; and (c) disposing the vapourised precipitate,or disposing the film-forming solution, on a substrate. Typically step(e) comprises depositing the vapourised precipitate on a substrate ordispersing the film-forming solution on a substrate.

In a first preferred embodiment, the process of the invention furthercomprises:

-   c) optionally washing the precipitate;-   d) dissolving the precipitate in an organic solvent to form a    film-forming solution; and-   e) dispersing the film-forming solution on a substrate.

In an alternative preferred embodiment of the invention, the processfurther comprises:

-   c′) optionally washing the precipitate;-   d′) vapourising the precipitate; and-   e′) depositing the vapourised precipitate on a substrate.

Without wishing to be bound by theory, it is speculated that the directproduct of step (b) typically contains not only the precipitate but alsospecies such as residual precursor compounds or side-products. It issuggested that by removing these additional components, for example bywashing, the thin film formed from the precipitate obtained in step (b)is less susceptible to regional stoichiometric variation of the A/M/Xmaterial caused by such sources of A, M and X.

The process of the invention, which provides a crystalline A/M/Xmaterial comprising a compound of formula [A]_(a)[M]_(b)[X]_(c), isapplicable to a wide variety of compounds. In some embodiments, thecompound of formula [A]_(a)[M]_(b)[X]_(c) is a compound of formula[A][M][X]₃. In some embodiments, the compound of formula[A]_(a)[M]_(b)[X]_(c) is a compound of formula [A]₂[M][X]₆. In yet otherembodiments, the compound of formula [A]_(a)[M]_(b)[X]_(c) is a compoundof formula [A]₄[M][X]₆.

The process of the invention is particularly valuable for producingA/M/X materials comprising multiple types of cation or anion, where itis even harder to control stoichiometry by conventional methods.Accordingly, in preferred aspects of the invention, [A] comprises two ormore different A cations and/or [X] comprises two or more different Xanions.

The invention further provides a thin film obtainable or obtained by aprocess for producing a thin film as defined herein.

The invention further provides an optoelectronic device, for example alight-emitting device or a photovoltaic device, comprising a thin filmas defined herein.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1(a) shows X-ray diffraction patterns of Cs_(1-x)Rb_(x)PbBr₃powders (with varying x).

FIG. 1(b) shows the X-ray diffraction patterns for thin films of thecorresponding materials.

FIG. 2(a) shows the absorption spectra of Cs_(1-x)Rb_(x)PbBr₃ powders(with varying x). FIG. 2(b) shows the absorption spectra of thin filmsof the corresponding materials.

FIGS. 3(a) and (b) show steady-state photoluminescence spectra ofCs_(1-x)Rb_(x)PbBr₃ powders;

FIGS. 3(c) and (d) show steady-state photoluminescence spectra for theircorresponding thin films.

FIG. 4(a) shows absorption spectra and FIG. 4(b)shows normalizedsteady-state photoluminescence spectra (the insert in 4(b) showsnon-normalised steady-state photoluminescence spectra) ofCs_(1-x)Rb_(x)PbBr₃ thin films fabricated from both a conventionalmethod and a method according to the invention (“invented method”).

FIG. 5 shows (a) absorption spectra, (b) steady-state photoluminescencespectra, and (c) normalized photoluminescence (“PL”) spectra ofCsPb(Br_(1-x)Cl_(x))₃ thin films.

FIG. 6 shows (a) absorption spectra, (b) steady-state photoluminescencespectra, and (c) normalized photoluminescence spectra ofCs_(1-x)Rb_(x)Pb(Br_(0.7)Cl_(0.3))₃ thin films.

FIG. 7 shows (a) steady-state photoluminescence spectra, and (b)normalized PL spectra, (c) absorption spectra and (d) X-ray diffractionpatterns of CsPb(Br_(0.7)Cl_(0.3))₃ thin films with different Cs/Pbratios.

FIG. 8 shows (a) powder X-ray diffraction patterns, (b) absorptionpatterns and (b) steady-state photoluminescence spectra ofCs_(x)PbBr_(y) produced using different Cs/Pb ratios in the A and Mprecursor solutions.

FIG. 9 shows (a) whole range powder X-ray diffraction patterns and (b) asub-range of the X-ray reflections from 2θ = 12 ° to 13.3° and 28° to30° of (Cs_(1-x)Rb_(x))₄PbBr₆ with different Rb to Cs fractions, x.

FIG. 10 shows (a) steady-state photoluminescence spectra and (b) thecorresponding normalised photoluminescence spectra of(Cs_(1-x)Rb_(x))₄PbBr₆ with different Rb concentrations.

FIG. 11 shows (a) whole range powder X-ray diffraction patterns and (b)a sub-range of the X-ray reflections from 2θ = 12 ° to 13° and 27° to30° of (Cs_(1-x)FA_(x))₄PbI₃Br₃.

FIG. 12 shows (a) steady-state photoluminescence spectra and (b) thecorresponding normalised photoluminescence spectra of(Cs_(1-x)FA_(x))₄PbI₃Br₃.

FIG. 13 shows an SEM image of a powder comprising CsPbBr₃ preparedaccording to the invention, with a 1:1 Cs:Pb ratio. The dark backgroundis a carbon tape coating the SEM sample holder. The brighter regions inthe image are the CsPbBr₃ powder. 13A shows this powder at magnificationof 1,000 times, while 13B shows this powder at a magnification of 10,000times.

FIG. 14 shows an SEM image of powder comprising CsPb(Br_(0.7)Cl_(0.3))₃.14A shows this powder at magnification of 1,000 times, while 14B showsthis powder at a magnification of 20,000 times.

FIG. 15 shows a powder comprising Cs₄PbBr₆, produced via the method ofthis invention from a starting solution Cs:Pb ratio of 10:1. 15A showsthis powder at magnification of 1,000 times, while 15B shows this powderat a magnification of 5,000 times.

FIG. 16(a) shows the normalised steady-state photoluminescence spectraof (Cs₁₋ _(x)Rb_(x))₄PbBr₆ powders with differing Rb content. FIG. 16(b)shows the PLQY of those powders, as function of exposure time undersimulated sun light (76 mW·cm⁻² at 60° C.).

FIG. 17(a) shows the normalised steady-state photoluminescence spectraof (Cs_(0.55)Rb_(0.45))₄PbBr₆ powders with a differing molar ratiobetween [A] (Cs⁺ and Rb⁺) and [M] (Pb²⁺). FIG. 17(b) shows the PLQY ofthose powders as function of exposure time under simulated sun light (76mW·cm⁻² at 60° C.).

FIG. 18 shows steady-state photoluminescence spectra of Cs₄PbBr₆ powdersobtained via different synthetic routes. FIG. 18(a) shows the spectra ofsuch powders obtained (i) by the method of the invention wherein DMSOwas used as the solvent for the lead precursor, and (ii) by a methodwherein hydrobromic acid was used as the solvent for the lead precursor.FIG. 18(b) shows the same spectra, normalised.

FIG. 19 shows X-ray diffraction patterns of Cs₄PbBr₆ powders. The middlespectrum is the spectrum of a Cs₄PbBr₆ powder obtained by the method ofthe invention wherein DMSO was used as the solvent for the leadprecursor; the top spectrum is the spectrum of a Cs₄PbBr₆ powderobtained by a method wherein hydrobromic acid was used as the solventfor the lead precursor; and the bottom spectrum is the calculated X-raydiffraction spectrum of Cs₄PbBr₆ in space group R-3cH.

FIG. 20 shows steady-state photoluminescence spectra of(Cs_(0.55)Rb_(0.45))₄PbBr₆ powders obtained by different syntheticroutes. FIG. 20(a) shows the spectra of such powders obtained (i) by themethod of the invention wherein DMSO was used as the solvent for thelead precursor, and (ii) by a method wherein hydrobromic acid was usedas the solvent for the lead precursor. FIG. 20(b) shows the samespectra, normalised.

FIG. 21 shows X-ray diffraction patterns of (Cs_(0.55)Rb_(0.45))₄PbBr₆powders. The middle spectrum is the spectrum of a(Cs_(0.55)Rb_(0.45))₄PbBr₆ powder obtained by the method of theinvention wherein DMSO was used as the solvent for the lead precursor;the top spectrum is the spectrum of a (Cs_(0.55)Rb_(0.45))₄PbBr₆ powderobtained by a method wherein hydrobromic acid was used as the solventfor the lead precursor; and the bottom spectrum is the calculated X-raydiffraction spectrum of Cs₄PbBr₆ in space group R-3cH.

FIG. 22 shows X-ray diffraction patterns of Rb₄PbBr₆ powders. The topspectrum is the spectrum of an Rb₄PbBr₆ powder obtained by the method ofthe invention wherein DMSO was used as the solvent for the leadprecursor; the bottom spectrum is the spectrum of a Rb₄PbBr₆ powderobtained by a method wherein hydrobromic acid was used as the solventfor the lead precursor.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter are described aspects of the process of the invention, andaspects of the products (e.g. powders or thin films) obtainable by theprocess of the invention. It should be understood that a description ofan aspect of the process of the invention relates also to the product ofthe invention insofar as it is relevant, and vice versa.

Definitions

The term “crystalline” as used herein indicates a crystalline compound,which is a compound having an extended 3D crystal structure. Acrystalline compound is typically in the form of crystals or, in thecase of a polycrystalline compound, crystallites (i.e. a plurality ofcrystals having particle sizes of less than or equal to 1 µm). Thecrystals together often form a layer. The crystals of a crystallinematerial may be of any size. Where the crystals have one or moredimensions in the range of from 1 nm up to 1000 nm, they may bedescribed as nanocrystals.

The terms “organic compound” and “organic solvent” as used herein havetheir typical meaning in the art and would readily be understood by theskilled person.

The term “disposing on”, as used herein, refers to the making availableor placing of one component on another component. The first componentmay be made available or placed directly on the second component, orthere may be a third component which intervenes between the first andsecond component. For instance, if a first layer is disposed on a secondlayer, this includes the case where there is an intervening third layerbetween the first and second layers. Typically, however, “disposing on”refers to the direct placement of one component on another.

The term “porous”, as used herein, refers to a material within whichpores are arranged. Thus, for instance, in a porous material the poresare volumes within the scaffold where there is no material. Pores in amaterial may include “closed” pores as well as open pores. A closed poreis a pore in a material which is a non-connected cavity, i.e. a porewhich is isolated within the material and not connected to any otherpore and which cannot therefore be accessed by a fluid to which thematerial is exposed. An “open pore” on the other hand, would beaccessible by such a fluid. The concepts of open and closed porosity arediscussed in detail in J. Rouquerol et al., “Recommendations for theCharacterization of Porous Solids”, Pure & Appl. Chem., Vol. 66, No. 8,pp.1739-1758, 1994. Open porosity, therefore, refers to the fraction ofthe total volume of the porous material in which fluid flow couldeffectively take place. It therefore excludes closed pores. The term“open porosity” is interchangeable with the terms “connected porosity”and “effective porosity”, and in the art is commonly reduced simply to“porosity”. The term “without open porosity”, as used herein, thereforerefers to a material with no effective porosity. The term “non-porous”as used herein, refers to a material without any porosity, i.e. withoutopen porosity and also without closed porosity.

The term “layer”, as used herein, refers to any structure which issubstantially laminar in form (for instance extending substantially intwo perpendicular directions, but limited in its extension in the thirdperpendicular direction). A layer may have a thickness which varies overthe extent of the layer. Typically, a layer has approximately constantthickness. The “thickness” of a layer, as used herein, refers to theaverage thickness of a layer. The thickness of layers may easily bemeasured, for instance by using microscopy, such as electron microscopyof a cross section of a film, or by surface profilometry for instanceusing a stylus profilometer.

The term “band gap”, as used herein, refers to the energy differencebetween the top of the valence band and the bottom of the conductionband in a material. The skilled person of course is readily able tomeasure the band gap of a semiconductor (including that of a perovskite)by using well-known procedures which do not require undueexperimentation. For instance, the band gap of a semiconductor can beestimated by constructing a photovoltaic diode or solar cell from thesemiconductor and determining the photovoltaic action spectrum.Alternatively the band gap can be estimated by measuring the lightabsorption spectra either via transmission spectrophotometry or by photothermal deflection spectroscopy. The band gap can be determined bymaking a Tauc plot, as described in Tauc, J., Grigorovici, R. & Vancu,a. Optical Properties and Electronic Structure of Amorphous Germanium.Phys. Status Solidi 15, 627-637 (1966) where the square of the productof absorption coefficient times photon energy is plotted on the Y-axisagainst photon energy on the x-axis with the straight line intercept ofthe absorption edge with the x-axis giving the optical band gap of thesemiconductor. Alternatively, the optical band gap may be estimated bytaking the onset of the incident photon-to-electron conversionefficiency, as described in [Barkhouse DAR, Gunawan O, Gokmen T, TodorovTK, Mitzi DB. Device characteristics of a 10.1% hydrazineprocessedCu2ZnSn(Se,S)4 solar cell. Progress in Photovoltaics: Research andApplications 2012; published online DOI: 10.1002/pip.1160.]

The term “semiconductor” or “semiconducting material”, as used herein,refers to a material with electrical conductivity intermediate inmagnitude between that of a conductor and a dielectric. A semiconductormay be an negative (n)-type semiconductor, a positive (p)-typesemiconductor or an intrinsic (i) semiconductor. A semiconductor mayhave a band gap of from 0.5 to 3.5 eV, for instance from 0.5 to 2.5 eVor from 1.0 to 2.0 eV (when measured at 300 K).

The term “n-type region”, as used herein, refers to a region of one ormore electron-transporting (i.e. n-type) materials. Similarly, the term“n-type layer” refers to a layer of an electron-transporting (i.e. ann-type) material. An electron-transporting (i.e. an n-type) materialcould be a single electron-transporting compound or elemental material,or a mixture of two or more electron-transporting compounds or elementalmaterials. An electron-transporting compound or elemental material maybe undoped or doped with one or more dopant elements.

The term “p-type region”, as used herein, refers to a region of one ormore hole-transporting (i.e. p-type) materials. Similarly, the term“p-type layer” refers to a layer of a hole-transporting (i.e. a p-type)material. A hole-transporting (i.e. a p-type) material could be a singlehole-transporting compound or elemental material, or a mixture of two ormore hole-transporting compounds or elemental materials. Ahole-transporting compound or elemental material may be undoped or dopedwith one or more dopant elements.

The term “electrode material”, as used herein, refers to any materialsuitable for use in an electrode. An electrode material will have a highelectrical conductivity. The term “electrode” as used herein indicates aregion or layer consisting of, or consisting essentially of, anelectrode material.

As used herein, the term “optionally substituted” means that the groupat issue may or may not bear substituents, i.e. it may be unsubstitutedor substituted. For instance, the group may bear 0, 1, 2, 3 or moresubstituents; typically 0, 1 or 2 substituents. The substituents maytypically be selected from substituted or unsubstituted C₁-C₂₀ alkyl,substituted or unsubstituted aryl (as defined herein), cyano, amino,C₁-C₁₀ alkylamino, di(C₁-C₁₀)alkylamino, arylamino, diarylamino,arylalkylamino, amido, acylamido, hydroxy, oxo, halo, carboxy, ester,acyl, acyloxy, C₁-C₂₀ alkoxy, aryloxy, haloalkyl, sulfonic acid,sulfhydryl (i.e. thiol, -SH), C₁-C₁₀ alkylthio, arylthio, and sulfonyl.

As used herein, an alkyl group can be a substituted or unsubstituted,linear or branched chain saturated radical, it is often a substituted oran unsubstituted linear chain saturated radical, more often anunsubstituted linear chain saturated radical. An alkyl group typicallycomprises 1 to 20 carbon atoms, usually 1 to 10 carbon atoms. A C₁₋₁₀alkyl group is an unsubstituted or substituted, straight or branchedchain saturated hydrocarbon radical, having from 1 to 10 carbon atoms. AC₁₋₆ alkyl group is an unsubstituted or substituted, straight orbranched chain saturated hydrocarbon radical, having from 1 to 6 carbonatoms. Typically it is, for example, methyl, ethyl, propyl, i-propyl,n-propyl, butyl, t-butyl, s-butyl, n-butyl, pentyl, or hexyl. Often analkyl group is a C₁₋₄ alkyl group. If the term “alkyl” is used without aprefix specifying the number of carbons anywhere herein, it generallyhas from 1 to 20 carbons (and this also applies to any other organicgroup referred to herein).

As used herein, an alkenyl group can be a substituted or unsubstituted,linear or branched chain unsaturated radical, it is often a substitutedor an unsubstituted linear chain unsaturated radical, more often anunsubstituted linear chain unsaturated radical. An alkenyl group cancomprise one or more carbon-carbon double bonds, for example one, two orthree double bonds. Typically, an alkenyl group comprises one doublebond. An alkenyl group typically comprises 2 to 20 carbon atoms, usually2 to 10 carbon atoms. A C₂₋₁₀ alkenyl group is an unsubstituted orsubstituted, straight or branched chain unsaturated hydrocarbon radical,having from 2 to 10 carbon atoms. A C₂₋₆ alkenyl group is anunsubstituted or substituted, straight or branched chain unsaturatedhydrocarbon radical, having from 2 to 6 carbon atoms. Typically it is,for example, vinyl, propenyl, prop-1-enyl, prop-2-enyl, butenyl,but-1-enyl, but-2-enyl, but-4-enyl, pentenyl, pent-1-enyl, pent-2-enyl,pent-3-enyl, pent-4-enyl, hexenyl, hex-1-enyl, hex-2-enyl, hex-3-enyl,hex-4-eny, or hex-5-enyl.

As used herein, a cycloalkyl group can be a substituted orunsubstituted, cyclic saturated radical, and it is often anunsubstituted cyclic saturated radical. A cycloalkyl group typicallycomprises from 3 to 20 carbon atoms. A C₃₋₁₀ cycloalkyl group is anunsubstituted or substituted, cyclic saturated hydrocarbon radical,having from 3 to 10 carbon atoms. A C₃₋₆ cycloalkyl group is anunsubstituted or substituted, cyclic saturated hydrocarbon radical,having from 3 to 6 carbon atoms Typically it is, for example,cyclopropyl, cyclobutyl, cyclopentyl or cyclohexyl.

As used herein, a cycloalkenyl group can be a substituted orunsubstituted, cyclic unsaturated radical, and it is often anunsubstituted cyclic unsaturated radical. A cycloalkenyl group typicallycomprises from 3 to 20 carbon atoms. A cycloalkenyl group may compriseone or more double bonds (dependent on the number of carbon atomspresent in the ring). A C₄₋₁₀ cycloalkenyl group is an unsubstituted orsubstituted, cyclic unsaturated hydrocarbon radical, having from 4 to 10carbon atoms. A C₄₋₁₀ cycloalkenyl group is an unsubstituted orsubstituted, cyclic unsaturated hydrocarbon radical, having from 3 to 6carbon atoms Typically it is, for example, cyclobutenyl, cyclopentenylor cyclohexenyl.

As used herein, an alkyloxy group is a radical comprising an alkyl groupas defined herein bound to an oxygen radical. That is, an alkoxy grouphas the formula -O-alkyl, wherein the alkyl radical is as definedherein. A C₁₋₁₀ alkyloxy group is therefore a radical of formula-O-alkyl wherein the alkyl radical is a C₁₋₁₀ alkyl radical as definedherein.

As used herein, an alkenyloxy group is a radical comprising an alkenylgroup as defined herein bound to an oxygen radical. That is, analkenyloxy group has the formula -O-alkenyl, wherein the alkenyl radicalis as defined herein. A C₁₋₁₀ alkenyloxy group is therefore a radical offormula -O-alkenyl wherein the alkenyl radical is a C₁₋₁₀ alkenylradical as defined herein.

The term “halide” as used herein indicates the singly charged anion ofan element in group VIII of the periodic table. “Halide” includesfluoride, chloride, bromide and iodide.

The term “halo” as used herein indicates a halogen atom. Exemplary halospecies include fluoro, chloro, bromo and iodo species.

As used herein, an aryl group is a substituted or unsubstituted,monocyclic or bicyclic aromatic group which typically contains from 6 to14 carbon atoms, often from 6 to 12 carbon atoms and preferably from 6to 10 carbon atoms in the ring portion. A C₆₋₁₂ aryl group is asubstituted or unsubstituted, monocyclic or bicyclic aromatic groupwhich contains from 6 to 12 carbon atoms. Examples include phenyl,naphthyl, indenyl and indanyl groups.

As used herein, an amino group is a radical of formula -NR₂, whereineach R is a substituent. R is usually selected from hydrogen, alkyl,alkenyl, cycloalkyl, or aryl, wherein each of alkyl, alkenyl, cycloalkyland aryl are as defined herein. Typically, each R is selected fromhydrogen, C₁₋₁₀ alkyl, C₂₋₁₀ alkenyl, and C₃₋₁₀ cycloalkyl. Preferably,each R is selected from hydrogen, C₁₋₆ alkyl, C₂₋₆ alkenyl, and C₃₋₆cycloalkyl. More preferably, each R is selected from hydrogen and C₁₋₆alkyl.

A typical amino group is an alkylamino group, which is a radical offormula -NR₂ wherein at least one R is an alkyl group as defined herein.A C₁₋₆ alkylamino group is an alkylamino group comprising 1 to 6 carbonatoms.

As used herein, an imino group is a radical of formula R₂C=N- or—C(R)═NR, wherein each R is a substituent. That is, an imino group is aradical comprising a C═N moiety, having the radical moiety either at theN atom or attached to the C atom of said C═N bond. R is as definedherein: that is, R is usually selected from hydrogen, alkyl, alkenyl,cycloalkyl, or aryl, wherein each of alkyl, alkenyl, cycloalkyl and arylare as defined herein. Typically, each R is selected from hydrogen,C₁₋₁₀ alkyl, C₂₋₁₀ alkenyl, and C₃₋₁₀ cycloalkyl. Preferably, each R isselected from hydrogen, C₁₋₆ alkyl, C₂₋₆ alkenyl, and C₃₋₆ cycloalkyl.More preferably, each R is selected from hydrogen and C₁₋₆ alkyl.

A typical imino group is an alkylimino group, which is a radical offormula R₂C=N- or —C(R)═NR wherein at least one R is an alkyl group asdefined herein. A C₁₋₆ alkylimino group is an alkylimino group whereinthe R substituents comprise from 1 to 6 carbon atoms.

The term “ester” as used herein indicates an organic compound of theformula alkyl-C(=O)-O-alkyl, wherein the alkyl radicals are the same ordifferent and are as defined herein. The alkyl radicals may beoptionally substituted.

The term “ether” as used herein indicates an oxygen atom substitutedwith two alkyl radicals as defined herein. The alkyl radicals may beoptionally substituted, and may be the same or different.

As used herein, the term “ammonium” indicates an organic cationcomprising a quaternary nitrogen. An ammonium cation is a cation offormula R¹R²R³R⁴N⁺. R¹, R², R³, and R⁴ are substituents. Each of R¹, R²,R³, and R⁴ are typically independently selected from hydrogen, or fromoptionally substituted alkyl, alkenyl, aryl, cycloalkyl, cycloalkenyland amino; the optional substituent is preferably an amino or iminosubstituent. Usually, each of R¹, R², R³, and R⁴ are independentlyselected from hydrogen, and optionally substituted C₁₋₁₀ alkyl, C₂₋₁₀alkenyl, C₃₋₁₀ cycloalkyl, C₃₋₁₀ cycloalkenyl, C₆₋₁₂ aryl and C₁₋₆amino; where present, the optional substituent is preferably an aminogroup; particularly preferably C₁₋₆ amino. Preferably, each of R¹, R²,R³, and R⁴ are independently selected from hydrogen, and unsubstitutedC₁₋₁₀ alkyl, C₂₋₁₀ alkenyl, C₃₋₁₀ cycloalkyl, C₃₋₁₀ cycloalkenyl, C₆₋₁₂aryl and C₁₋₆ amino. In a particularly preferred embodiment, R¹, R², R³,and R⁴ are independently selected from hydrogen, C₁₋₁₀ alkyl, and C₂₋₁₀alkenyl and C₁₋₆ amino. Further preferably, R¹, R², R³, and R⁴ areindependently selected from hydrogen, C₁₋₆ alkyl, C₂₋₆ alkenyl and C₁₋₆amino.

As used herein, the term “iminium” indicates an organic cation offormula (R¹R²C=NR³R⁴)⁺, wherein R¹, R², R³, and R⁴ are as defined inrelation to the ammonium cation. Thus, in a particularly preferredembodiment, of the iminium cation, R¹, R², R³, and R⁴ are independentlyselected from hydrogen, C₁₋₁₀ alkyl, C₂₋₁₀ alkenyl and C₁₋₆ amino. In afurther preferable embodiment of the iminium cation, R¹, R², R³, and R⁴are independently selected from hydrogen, C₁₋₆ alkyl, C₂₋₆ alkenyl andC₁₋₆ amino.

The term “perovskite”, as used herein, refers to a material with athree-dimensional crystal structure related to that of CaTiO₃ or amaterial comprising a layer of material, which layer has a structurerelated to that of CaTiO₃. A material with a three-dimensional crystalstructure related to that of CaTiO₃ may be referred to as being aperovskite having a “3D perovskite structure”, or as being a “3Dperovskite”. The structure of CaTiO₃ can be represented by the formulaABX₃, wherein A and B are cations of different sizes and X is an anion.In the unit cell, the A cations are at (0,0,0), the B cations are at (½,½, ½) and the X anions are at (½, ½, 0). The A cation is usually largerthan the B cation. The skilled person will appreciate that when A, B andX are varied, the different ion sizes may cause the structure of theperovskite material to distort away from the structure adopted by CaTiO₃to a lower-symmetry distorted structure. The symmetry will also be lowerif the material comprises a layer that has a structure related to thatof CaTiO₃. Materials comprising a layer of perovskite material are wellknown. For instance, the structure of materials adopting the K₂NiF₄-typestructure comprises a layer of perovskite material. These are referredto in the art as “2D layered perovskites”, being different in structurefrom the above-mentioned 3D perovskites. 2D layered perovskites can berepresented by the formula [A]₂[M][X]₄, wherein [A] is at least onecation, [M] is at least one cation of a different size to the cation(s)A and [X] is at least one anion.

The skilled person will appreciate that a 3D perovskite material can berepresented by the formula [A][B][X]₃, wherein [A] is at least onecation, [B] is at least one cation and [X] is at least one anion. Whenthe perovskite comprise more than one A cation, the different A cationsmay distributed over the A sites in an ordered or disordered way. Whenthe perovskite comprises more than one B cation, the different B cationsmay distributed over the B sites in an ordered or disordered way. Whenthe perovskite comprise more than one X anion, the different X anionsmay distributed over the X sites in an ordered or disordered way. Thesymmetry of a perovskite comprising more than one A cation, more thanone B cation or more than one X cation, will be lower than that ofCaTiO₃.

The term “metal halide perovskite”, as used herein, refers to aperovskite, the formula of which contains at least one metal cation andat least one halide anion.

The term “mixed halide perovskite” as used herein refers to a perovskiteor mixed perovskite which contains at least two types of halide anion.

The term “mixed cation perovskite” as used herein refers to a perovskiteof mixed perovskite which contains at least two types of A cation.

The term “hexahalometallate”, as used herein, refers to a compound whichcomprises an anion of the formula [MX₆]^(n-), wherein M is a metal atom,each X is independently a halide anion and n is an integer from 1 to 4.

The term “optoelectronic device”, as used herein, refers to deviceswhich source, control, detect or emit light. Light is understood toinclude any electromagnetic radiation. Examples of optoelectronicdevices include photovoltaic devices, photodiodes, solar cells,photodetectors, phototransistors, photomultipliers, photoresistors,chromogenic devices, light -sensitive transistors, light emittingdevices, light emitting diodes and charge injection lasers.

The term “consisting essentially of” refers to a composition comprisingthe components of which it consists essentially as well as othercomponents, provided that the other components do not materially affectthe essential characteristics of the composition. Typically, acomposition consisting essentially of certain components will comprisegreater than or equal to 95 wt% of those components or greater than orequal to 99 wt% of those components.

Process

The invention provides a process for producing a crystalline A/M/Xmaterial, which crystalline A/M/X material comprises a compound offormula:

-   wherein:    -   [A] comprises one or more A cations;    -   [M] comprises one or more M cations which are metal or metalloid        cations;    -   [X] comprises one or more X anions, which are typically one or        more halide anions;    -   a is a number from 1 to 6;    -   b is a number from 1 to 6; and    -   c is a number from 1 to 18,-   the process comprising:    -   a) contacting an aqueous solution comprising an A precursor and        an aqueous solvent with an organic solution comprising an M        precursor and an organic solvent; and    -   b) allowing a precipitate to form when the said aqueous and        organic solutions are contacted.

In the process of the invention, “contacting” an aqueous solution withan organic solution means that the said solutions are brought intocontact. There is no particular limitation as to how this step isperformed. For instance, the aqueous solution may be added to theorganic solution, or the organic solution may be added to the aqueoussolution. For example, the process may comprise providing the organicsolution in a reaction vessel and adding the aqueous solution to thereaction vessel, or providing the aqueous solution in a reaction vesseland adding the organic solution to the reaction vessel. In otherembodiments, the aqueous solution and the organic solution may be addedto one another, for example by providing both solutions to a reactionvessel at the same time.

The contacting step may be performed rapidly or slowly. For instance,the contacting step may be performed by adding the aqueous solutiongradually (e.g. dropwise) to the organic solution, or by adding theorganic solution gradually (e.g. dropwise) to the aqueous solution.

The aqueous solution typically comprises at least 50% water by weight ofthe aqueous solution. Preferably, the aqueous solution comprises atleast 60% water by weight of the aqueous solution, for example at least70% or at least 80% or at least 90% water by weight of the aqueoussolution.

The aqueous solution comprises an A precursor and an aqueous solvent. Ofcourse, the aqueous solution may comprise other species. For example,the aqueous solution may comprise dissolved compounds (typicallydissolved ionic compounds) such as impurities, stabilising agents, pHcontrolling agents and so on. Moreover, the aqueous solution maycomprise additional solvents.

In some embodiments, the aqueous solution is substantially free oforganic solvents. The term “organic solvents” takes its usual meaning inthe art. For example, in some embodiments the aqueous solution comprises1% or less of organic solvents by weight of the aqueous solution,preferably 0.5% or less of organic solvents by weight of the aqueoussolution; e.g. 0.1% or less, or 0.01% or less of organic solvents byweight of the aqueous solution.

As the skilled reader will appreciate, by the term solution it isimplied that the aqueous solution is a liquid. However, the skilledreader will also appreciate that in practice the aqueous solution maycomprise solid impurities in small quantities. A typical example of suchsolid impurities includes small quantities of one or more undissolved Aprecursor compound(s). However, it is preferred that these solidimpurities are minimised. Accordingly, it is preferred that the aqueoussolution comprises less than 1% of solid by weight of the aqueoussolution, preferably 0.5% or less by weight of solid by weight of theaqueous solution; e.g. 0.1% or less, or 0.01% or less by weight of solidby weight of the aqueous solution.

The aqueous solution comprises an A precursor and an aqueous solvent. Asis discussed in more detail below, an A precursor is a compoundcomprising one or more A cations present in [A]. Where [A] (that is, [A]in the compound of formula [A]_(a)[M]_(b)[X]_(c)) comprises only onetype of A cation, only one A precursor is necessary in the process ofthe invention.

Where [A] comprises two or more cations, a number of options areavailable in the process of the invention. For example, if an Aprecursor comprising the two or more A cations is available, that Aprecursor comprising the two or more A cations present in [A] may bepresent in the aqueous solution. Alternatively, step (a) of the processmay involve contacting a plurality of aqueous solutions, each comprisingan A precursor and an aqueous solvent, with an organic solutioncomprising an M precursor and an organic solvent. In yet anotheralternative, the aqueous solution utilised in step (a) of the processmay comprise a plurality of A precursors.

For simplicity, it may be preferred in the process of the invention thata single aqueous solution is contacted with an organic solution. Inparticular, it may be preferred in the process of the invention that asingle aqueous solution is contacted with a single organic solution.

Accordingly, it may be preferred that the aqueous solution compriseseach of the one or more A cations present in [A]. As discussed herein,where [A] comprises two or more A cations, an aqueous solutioncomprising all A cations present in [A] may be achieved by an aqueoussolution comprising one A precursor, wherein the said A precursorcomprises each of the one or more A cations present in [A]; or by anaqueous solution comprising a plurality of A precursors, wherein each Aprecursor comprises one or more A cations present in [A] and the saidplurality of A precursors comprises all of the A cations present in [A].

The organic solution typically comprises at least 50% organic solvent(s)by weight of the organic solution. Preferably, the organic solutioncomprises at least 60% organic solvents by weight of the organicsolution, for example at least 70% or at least 80% or at least 90%organic solvents by weight of the organic solution.

The organic solution comprises an M precursor and an organic solvent. Ofcourse, the organic solution may comprise other species. For example,the organic solution may comprise dissolved compounds (which may forinstance be dissolved ionic compounds or organic compounds) such asimpurities, stabilising agents, pH controlling agents and so on.Moreover, the organic solution may comprise additional solvents.

In some embodiments, the organic solution is substantially free ofwater. For example, in some embodiments the organic solution comprises5% or less of water by weight of the organic solution, preferably 1% orless of water by weight of the organic solution; e.g. 0.5% or less, or0.1% or less of water by weight of the organic solution.

As the skilled reader will appreciate, by the term solution it isimplied that the organic solution is a liquid. However, the skilledreader will also appreciate that in practice the organic solution maycomprise solid impurities in small quantities. A typical example of suchsolid impurities includes small quantities of one or more undissolved Mprecursor compound(s). However, it is preferred that these solidimpurities are minimised. Accordingly, it is preferred that the organicsolution comprises less than 1% of solid by weight of the organicsolution, preferably 0.5% or less by weight of solid by weight of theorganic solution; e.g. 0.1% or less, or 0.01% or less by weight of solidby weight of the organic solution.

The organic solution comprises an M precursor and an organic solvent. Asis discussed in more detail below, an M precursor is a compoundcomprising one or more M cations present in [M]. Where [M] (that is, [M]in the compound of formula [A]_(a)[M]_(b)[X]_(c)) comprises only onetype of M cation, only one M precursor is necessary in the process ofthe invention.

Where [M] comprises two or more cations, a number of options areavailable in the process of the invention. For example, if an Mprecursor comprising the two or more M cations is available, that Mprecursor comprising the two or more M cations present in [M] may bepresent in the organic solution. Alternatively, step (a) of the processmay involve contacting a plurality of organic solutions, each comprisingan M precursor and an organic solvent, with an aqueous solutioncomprising an A precursor and an organic solvent. In yet anotheralternative, the organic solution utilised in step (a) of the processmay comprise a plurality of M precursors.

For simplicity, it may be preferred in the process of the invention thata single organic solution is contacted with an aqueous solution. Inparticular, it may be preferred in the process of the invention that asingle aqueous solution is contacted with a single organic solution.

Accordingly, it may be preferred that the organic solution compriseseach of the one or more M cations present in [M]. As discussed herein,where [M] comprises two or more M cations, an organic solutioncomprising all M cations present in [M] may be achieved by an organicsolution comprising one M precursor, wherein the said M precursorcomprises each of the one or more M cations present in [M]; or by anorganic solution comprising a plurality of M precursors, wherein each Mprecursor comprises one or more M cations present in [M] and the saidplurality of M precursors comprises all of the M cations present in [M].Thus, in a preferred embodiment, the process comprises:

-   a) contacting an aqueous solution comprising one or more A    precursors and an aqueous solvent with an organic solution    comprising one or more M precursors and an organic solvent, wherein    the one or more A precursors together comprise all the cations    present in [A] and the one or more M precursors together comprise    all the cations present in [M]; and-   b) allowing a precipitate to form when the said aqueous and organic    solutions are contacted.

As regards the source of X anions in the process of the invention, itmay not be necessary to provide a separate X precursor in the process ofthe invention. This is because in some embodiments, the A precursor (orwhere the process involves a plurality of A precursors, at least one ofthem) and/or the M precursor (or where the process involves a pluralityof M precursors, at least one of them) is a halide salt. In a preferredembodiment, the A precursor (or where present the plurality of Aprecursors) and the M precursor (or where present the plurality of Mprecursors) together comprise each of the X cations present in [X].

In some embodiments, a compound comprising an X anion is provided duringthe process of the invention. Where [X] comprises a single X anion, acompound comprising that X anion may be provided. Where [X] comprises aplurality of X anions, a compound comprising one or more, e.g. all of,the said plurality of X anions may be provided during the process of theinvention. Equally, a plurality of compounds each comprising one or moreof the said plurality of X anions may be provided during the process ofthe invention. A compound which comprises one or more X anions but doesnot comprise any A cations or M cations may be referred to as an Xprecursor. Typically, an X precursor is a hydrohalic acid comprising anX anion; that is, a compound of formula HX′.

In some embodiments, an X precursor may be provided in a separatesolution. For instance, in some embodiments the process may comprise:

-   a) contacting an aqueous solution comprising an A precursor and an    aqueous solvent with an organic solution comprising an M precursor    and an organic solvent, and also with a further solution comprising    a further solvent and an X precursor; and-   b) allowing a precipitate to form when the said aqueous solution,    organic solution and further solution are contacted.

The further solvent is not particularly limited. It may be an aqueoussolvent or an organic solvent. Where it is an aqueous solvent, it may ormay not be the same aqueous solvent as is present in the aqueoussolution. Where it is an organic solvent, it may or may not be the sameorganic solvent as is present in the organic solution. The furthersolution may comprise one or more solvent(s) in addition to the furthersolvent.

However, for simplicity it may be preferred that the number of solutionscontacted during the reaction is small. Accordingly, where an Xprecursor is used in the process of the invention it may be preferredthat said X precursor is present in at least one of the aqueous solutionand the organic solution. For example, the process of the invention maycomprise:

-   a) contacting an aqueous solution comprising an A precursor and an    aqueous solvent with an organic solution comprising an M precursor    and an organic solvent, wherein the aqueous solution and/or the    organic solution comprises an X precursor; and-   b) allowing a precipitate to form when the said aqueous solution and    organic solution are contacted.

Typically, the organic solution comprises the one or more X precursors(where used). Accordingly, in a preferred embodiment, the organicsolution comprises an X precursor. In a particularly preferredembodiment, the organic solution comprises a hydrohalic acid.

For example, the organic solution may comprise one or more hydrohalicacids of formula HX′, wherein each X′ is the halide anion X or one ofthe halide anions X (which are present in [X]). Preferably, the organicsolution comprises a hydrohalic acid of formula HX′ corresponding toeach of the X anions present in [X].

The process of the invention thus comprises:

-   a) contacting: one or more aqueous solutions, each comprising one or    more A precursors, one or more organic solutions, each comprising    one or more M precursors, and one or more X anions; and-   b) allowing a precipitate to form when the one or more A precursors,    one or more M precursors and one or more X anions are contacted.

When the A, M and (where present) X precursors are brought into contact,they can form an [A]_(a)[M]_(b)[X]_(c) compound which generally has poorsolubility in both the aqueous solvent and the organic solvent.Accordingly, a precipitate comprising the [A]_(a)[M]_(b)[X]_(c) compoundgenerally forms regardless of the relative quantities of A, M and X ionsin the initial solutions and in the reaction mixture.

However, it may assist with control of the stoichiometry of the[A]_(a)[M]_(d)[X]_(c) compound to ensure that the A, M and X ionscontacted in step (a) of the process are present in molar ratiosrelative to one another that approximately reflect the relative molarratios of those ions in the desired [A]_(a)[M]_(d)[X]_(c) product.

Often, therefore the molar ratio [A]:[M] in the aqueous solution orsolutions and the organic solution or solutions is approximately equalthe [A]:[M] ratio in the product. That is, the total molar concentrationof A cation(s) in the aqueous solution(s) divided by the total molarconcentration of the M cation(s) in the organic solution(s) is generallyequal to a divided by b, wherein a and b are as in the[A]_(a)[M]_(b)[X]_(c) compound.

In a preferred embodiment, where the [A]_(a)[M]_(b)[X]_(c) compoundcomprises two or more A cations, in the process of the invention theaqueous solution comprises the two or more A cations in a molar ratioapproximately equivalent to their molar ratio in the desired product.The two or more different A cations may be indicated by, for instance,A^(I) and A^(II). Accordingly, by way of example, where the[A]_(a)[M]_(b)[X]_(c) compound comprises A^(I) and A^(II) in a 3:1 molarratio (that is, the [A]_(a)[M]_(b)[X]_(c) compound may be [A^(I)_(0.75)A^(II) _(0.25)]_(a)[M]_(b)[X]_(c)), the aqueous solution maycomprise A^(I) cations and A^(II) cations in a molar ratio of about 3:1.For instance, the invention provides a process for producing acrystalline A/M/X material, which crystalline A/M/X material comprises acompound of formula:

-   wherein:    -   [A] comprises two cations A^(I) and A^(II);    -   [M] comprises one or more M cations which are metal or metalloid        cations;    -   [X] comprises one or more X anions which are halide anions;    -   a is a number from 1 to 6;    -   b is a number from 1 to 6; and    -   c is a number from 1 to 18,-   the process comprising:    -   a) contacting an aqueous solution comprising an A^(I) precursor,        an A^(II) precursor and an aqueous solvent with an organic        solution comprising an M precursor and an organic solvent; and    -   b) allowing a precipitate to form when the said aqueous and        organic solutions are contacted,-   wherein the molar ratio of A^(I):A^(II) in the aqueous solution is    approximately the same as the molar ratio of A^(I):A^(II) in the    compound of formula [A]_(a)[M]_(b)[X]_(c).

Similarly, in another preferred embodiment, where the[A]_(a)[M]_(b)[X]_(c) compound comprises two or more M cations, in theprocess of the invention the organic solution comprises the two or moreM cations in a molar ratio approximately equivalent to their molar ratioin the desired product. The two or more different M cations may beindicated by, for instance, M^(I) and M^(II). Accordingly, by way ofexample, where the [A]_(a)[M]_(b)[X]_(c) compound comprises M^(I) andM^(II) in a 1:1 molar ratio (that is, the [A]_(a)[M]_(b)[X]_(c) compoundmay be [A]_(a)[M^(I) _(0.5)M^(II) _(0.5)]_(b)[X]_(c)), the organicsolution may comprise M^(I) cations and M^(II) cations in a molar ratioof about 1:1.

Also similarly, where the [A]_(a)[M]_(b)[X]_(c) compound comprises twoor more X anions, in the process of the invention the molar ratio of thetwo or more X anions in the aqueous solution and the organic solutioncombined is generally approximately equivalent to the molar ratio of theX anions in the desired product. The two or more different X cations maybe indicated by, for instance, X^(I) and X^(II). Accordingly, by way ofexample, where the [A]_(a)[M]_(b)[X]_(c) compound comprises X^(I) andX^(II) in a 1:4 molar ratio (that is, the [A]_(a)[M]_(b)[X]_(c) compoundmay be [A]_(a)[M]_(b)[X^(I) _(0.2)X^(II) _(0.8)]_(c)), the molar ratioof X^(I) anions to X^(II) anions in the aqueous solution and the organicsolution together may be about 1:4.

In some embodiments of the invention, therefore, the relative molaramounts of each of said one or more A cations, said one or more Mcations and said one or more X anions in the aqueous solution togetherwith the organic solution are equal to the relative molar amounts ofeach of said one or more A cations, said one or more M cations and saidone or more X anions in the compound of formula [A]_(a)[M]_(b)[X]_(c).It is preferred in this embodiment that the process involves one aqueoussolution comprising the or each A precursor and one organic solutioncomprising the or each M precursor, and optionally the or each Xprecursor.

In a preferred aspect of this embodiment, [X] comprises a first X anionand a second X anion and the molar ratio of the first X anion to thesecond X anion is the same in each of:

-   (i) the aqueous solution;-   (ii) the organic solution; and-   (iii) the compound of formula [A]_(a)[M]_(b)[X]_(c).

The process of contacting an organic solution and an aqueous solution(and where necessary any additional solutions) forms a reaction mixture.The “reaction mixture” comprises the aqueous solution and the organicsolution of step (a) in contact. The reaction mixture may optionallycomprise additional components, for instance a further solutioncomprising an X precursor. Immediately after an organic solution and anaqueous solution are brought into contact to form a reaction mixture,the reaction mixture will typically comprise an A precursor and an Mprecursor. After some precipitation has occurred, the reaction mixturewill comprise a precipitate comprising solid crystalline A/M/X material.Additionally, the quantity of A precursor and M precursor in thereaction mixture will be reduced compared to their amounts at the timewhen the aqueous solution and organic solution were brought intocontact.

In the process of the invention, a precipitate is allowed to form whenthe said aqueous and organic solutions are contacted. “Allowing” aprecipitate to form may not require action. Simply bringing theprecursors into contact by contacting an organic solution and an aqueoussolution (and where necessary any additional solutions) is generallysufficient to cause a precipitate to form. However, in some embodiments,“allowing” a precipitate to form may optionally comprise agitating thereaction mixture, for instance mixing the aqueous solution and theorganic solution. “Allowing” a precipitate to form may optionallycomprise heating the reaction mixture.

The process of the invention may therefore comprise:

-   a) contacting an aqueous solution comprising an A precursor and an    aqueous solvent with an organic solution comprising an M precursor    and an organic solvent to form a reaction mixture comprising said    aqueous solution and said organic solution; and-   b) allowing a precipitate to form when the said aqueous and organic    solutions are contacted, which process optionally comprises    agitating the reaction mixture.

Suitable methods of agitating the reaction mixture would be well knownto the person skilled in the art, for instance stirring the reactionmixture e.g. with a stirrer bar.

During the step of allowing a precipitate to form, the process maycomprise heating the reaction mixture. For example step (b) may compriseheating the reaction mixture to a temperature of from 25° C. to 120° C.,preferably to a temperature of from 30° C. to 100° C.; more preferablyto a temperature of from 40° C. to 80° C. Heating the reaction mixturemay be desirable where one or more of the precursors involved has poorsolubility. However, heating the reaction mixture may discourageprecipitation of the A/M/X product and so in some embodiments step (b)does not comprise heating the reaction mixture.

The precipitate formed in step (b) comprises crystals of a crystallineA/M/X material. The said crystals are typically very small. Forinstance, the said crystals generally have a diameter of 1000 nm orless, for example 100 nm or less, or 10 nm or less. By “diameter” ismeant the diameter of the largest sphere that fits entirely inside thecrystal. Crystal size can conveniently be measured by standardtechniques such as SEM, TEM or DLS (that is, dynamic light scattering).Typically crystal size is measured by SEM.

In some embodiments, therefore, at least 80% or at least 90% of thecrystals of the precipitate have a diameter of 100 nm or less, andpreferably at least 80% or at least 90% of the crystals of theprecipitate have a diameter of 10 nm or less.

In some embodiments, the precipitate comprises crystals having adiameter of 2 nm to 1000 nm; e.g. from 10 nm to 100 nm. For instance, insome embodiments at least 80% or at least 90% of the crystals of theprecipitate have a diameter of from 2 nm to 1000 nm; preferably at least80% or at least 90% of the crystals of the precipitate have a diameterof 10 nm to 100 nm.

In some embodiments, the process may comprise reducing the size of theprecipitate after it is formed. This step may be performed after step(b) and/or after step (c) or step (c′). Typical methods for reducing thesize of the precipitate are mechanical methods, for example ballgrinding. Ball grinding is described in “Mechanochemical synthesis ofadvanced nanomaterials for catalytic applications”, Xu et al., Chem.Commun., 2015, 51, 6698.

After grinding, the precipitate may comprise somewhat smallerparticulates. For instance, after grinding, at least 80% or at least 90%of the crystals of the precipitate typically have a diameter of from 2to 100 nm, e.g. from 5 to 50 nm.

The precipitate may be in the form of a powder. Exemplary powders inaccordance with the invention are shown in FIGS. 13, 14 and 15 .

Due to the small size of the crystals in the precipitate, theprecipitate may often be referred to as a powder. The precipitate mayoften be referred to as a nanopowder, if the average particle size isfrom 5 to 50 nm.

Preparation of Aqueous and Organic Solutions

The process of the invention involves at least one organic solution andat least one aqueous solution. In some embodiments of the invention, theprocess comprises preparing the aqueous solution, or preparing theorganic solution, or preparing the aqueous solution and the organicsolution.

The aqueous solution comprises an aqueous solvent and an A precursor.Typically, therefore, the aqueous solution can be prepared by dissolvingthe said A precursor in the said aqueous solvent.

In some embodiments, the compound of formula [A]_(a)[M]_(b)[X]_(c)comprises two or more A cations. In a preferred aspect of thisembodiment, the process comprising contacting a single aqueous solutioncomprising one or more A precursors as necessary to provide each of theA cations with an organic solution as defined herein.

Where a single A precursor comprising each of the said two or more Acations is available, an aqueous solution for use in the invention maybe prepared by dissolving that A precursor comprising each of the two ormore A cations in an aqueous solvent. However, where such a precursor isnot available, it may be necessary to prepare an aqueous solutioncomprising two or more A precursors. This may be done by dissolving aplurality of A precursors in a single solvent (typically an aqueoussolvent). Alternatively, this may be done by dissolving one or more Aprecursors in each of two or more solvents, and subsequently combiningthe resulting solutions.

In some embodiments, therefore, wherein [A] comprises two or moredifferent A cations, the process comprises prior to step (a), a step ofpreparing an aqueous solution by combining a solution of a first Aprecursor with a solution of a second A precursor, wherein:

-   the first A precursor comprises a first A cation; and-   the second A precursor comprises a second A cation.

The first A precursor comprises “a first A cation”, which mayconveniently referred to using the notation A^(I). Accordingly, thefirst A precursor or “A^(I) precursor” comprises a cation A^(I). Thesecond A precursor comprises “a second A cation”, which may convenientlybe referred to using the notation A^(II) to indicate an A cationdifferent to A^(I). Accordingly, the second A precursor or A^(II)precursor comprises a cation A^(II). The solution of the first Aprecursor is typically an aqueous solution as defined herein. Thesolution of the second A precursor is also typically an aqueous solutionas defined herein.

Each solution of each A precursor is typically prepared by dissolvingthe A precursor in an aqueous solvent.

The invention therefore provides a process for producing a crystallineA/M/X material, which crystalline A/M/X material comprises a compound offormula:

-   wherein:    -   [A] comprises an A^(I) cation and an A^(II) cation;    -   [M] comprises one or more M cations which are metal or metalloid        cations;    -   [X] comprises one or more X anions which are halide anions;    -   a is a number from 1 to 6;    -   b is a number from 1 to 6; and    -   c is a number from 1 to 18,-   the process comprising combining a first aqueous solution comprising    an A^(I) precursor and a first aqueous solvent with a second aqueous    solution comprising an A^(II) precursor and a second aqueous solvent    (which may be the same as or different to the first aqurous solvent)    to form an aqurous solution, and:    -   a) contacting the aqueous solution with an organic solution        comprising an M precursor and an organic solvent; and    -   b) allowing a precipitate to form when the said aqueous and        organic solutions are contacted.

In some embodiments, the compound of formula [A]_(a)[M]_(b)[X]_(c)comprises two or more M cations. In a preferred aspect of thisembodiment, the process comprising contacting a single organic solutioncomprising one or more M precursors as necessary to provide each of theM cations with an aqueous solution as defined herein.

Where a single M precursor comprising each of the said two or more Mcations is available, an organic solution for use in the invention maybe prepared by dissolving that M precursor comprising each of the two ormore M cations in an organic solvent. However, where such a precursor isnot available, it may be necessary to prepare an organic solutioncomprising two or more M precursors. This may be done by dissolving aplurality of M precursors in a single solvent (typically an organicsolvent). Alternatively, this may be done by dissolving one or more Mprecursors in each of two or more solvents, and subsequently combiningthe resulting solutions.

In some embodiments, therefore, wherein [M] comprises two or moredifferent M cations, the process comprises prior to step (a), a step ofpreparing an organic solution by combining a solution of a first Mprecursor with a solution of a second M precursor, wherein:

-   the first M precursor comprises a first M cation; and-   the second M precursor comprises a second M cation.

The first M precursor comprises “a first M cation”, which mayconveniently referred to using the notation M^(I). Accordingly, thefirst M precursor or “M^(I) precursor” comprises a cation M^(I). Thesecond M precursor comprises “a second M cation”, which may convenientlybe referred to using the notation M^(II) to indicate an M cationdifferent to M^(I). Accordingly, the second M precursor or “M^(II)precursor” comprises a cation M^(II). The solution of the first Mprecursor is typically an organic solution as defined herein. Thesolution of the second M precursor is also typically an organic solutionas defined herein.

Each solution of each M precursor is typically prepared by dissolvingthe M precursor in an organic solvent. Each of the organic solvents maycomprise one or more X precursors. For instance, each of the organicsolvents may comprise a hydrohalic acid of formula HX′, where X′ is oneof the X anions present in [X].

The invention therefore provides a process for producing a crystallineA/M/X material, which crystalline A/M/X material comprises a compound offormula:

-   wherein:    -   [A] comprises one or more A cations;    -   [M] comprises an M^(I) cation and an M^(II) cation which are        both metal or metalloid cations;    -   [X] comprises one or more X anions which are halide anions;    -   a is a number from 1 to 6;    -   b is a number from 1 to 6; and    -   c is a number from 1 to 18,-   the process comprising combining a first organic solution comprising    an M^(I) precursor, a first organic solvent and optionally a    hydrohalic acid with a second organic solution comprising an M^(II)    precursor, a second organic solvent (which may be the same as or    different to the first organic solvent) and optionally a hydrohalic    acid to form an organic solution, and:    -   a) contacting an aqueous solution comprising an A precursor and        an aqueous solvent with the said organic solution; and    -   b) allowing a precipitate to form when the said aqueous and        organic solutions are contacted.

In one embodiment, the invention provides a process for producing acrystalline A/M/X material, which crystalline A/M/X material comprises acompound of formula:

-   wherein:    -   [A] comprises an A^(I) cation and an A^(II) cation;    -   [M] comprises an M^(I) cation and an M^(II) cation which are        both metal or metalloid cations;    -   [X] comprises one or more X anions which are halide anions;    -   a is a number from 1 to 6;    -   b is a number from 1 to 6; and    -   c is a number from 1 to 18,-   the process comprising:    -   combining a first organic solution comprising an M^(I)        precursor, a first organic solvent and optionally a hydrohalic        acid with a second organic solution comprising an M^(II)        precursor, a second organic solvent (which may be the same as or        different to the first organic solvent) and optionally a        hydrohalic acid to form an organic solution; combining a first        aqueous solution comprising an A^(I) precursor and a first        aqueous solvent with a second aqueous solution comprising an        A^(II) precursor and a second aqueous solvent (which may be the        same as or different to the first aqueous solvent) to form an        aqueous solution, and:        -   a) contacting the said aqueous solution with the said            organic solution; and        -   b) allowing a precipitate to form when the said aqueous and            organic solutions are contacted.

In some embodiments, the compound of formula [A]_(a)[M]_(b)[X]_(c)comprises two or more X anions. In a preferred aspect of thisembodiment, the process comprises contacting a single organic solutionwith a single aqueous solution which together contain the relevant Xanions. In this embodiment, it may optionally be preferred that theaqueous solution and/or the organic solution each comprise the two ormore X anions present in the compound of formula [A]_(a)[M]_(b)[X]_(c).This may assist with finely balancing the stoichiometry of theprecipitate.

In such an embodiment, where a single A precursor comprising each of thesaid two or more X anions is available, an aqueous solution for use inthe invention may be prepared by dissolving that A precursor comprisingeach of the two or more X anions in an aqueous solvent. Similarly, wherea single M precursor comprising each of the said two or more X anions isavailable, an organic solution for use in the invention may be preparedby dissolving that M precursor comprising each of the two or more Xanions in an organic solvent. However, where such precursors are notavailable, it may be necessary to prepare an aqueous solution and/or andorganic solution comprising two or more A precursors or M precursorsrespectively. In the case of the aqueous solution, this may be done bydissolving a plurality of A precursors, each comprising one or more Xanions, in a single solvent (typically an aqueous solvent).Alternatively, this may be done by dissolving one or more A precursorseach comprising one or more X anions in each of two or more solvents,and subsequently combining the resulting solutions. Similarly, in thecase of the organic solution, this may be done by dissolving a pluralityof M precursors (each comprising one or more X anions) in a singlesolvent (typically an organic solvent). Alternatively, this may be doneby dissolving one or more M precursors (each comprising one or more Xanions) in each of two or more solvents, and subsequently combining theresulting solutions.

For example, the invention provides a process for producing acrystalline A/M/X material, which crystalline A/M/X material comprises acompound of formula:

-   wherein:    -   [A] comprises an A cation;    -   [M] comprises an M cation which are both metal or metalloid        cations;    -   [X] comprises an X^(I) anion and an X^(II) anion which are        halide anions;    -   a is a number from 1 to 6;    -   b is a number from 1 to 6; and    -   c is a number from 1 to 18,-   the process comprising:    -   combining a first organic solution comprising an M precursor        which contains an X^(I) anion, a first organic solvent and        optionally a hydrohalic acid with a second organic solution        comprising an M precursor which contains an X^(II) anion, a        second organic solvent (which may be the same as or different to        the first organic solvent) and optionally a hydrohalic acid to        form an organic solution;    -   combining a first aqueous solution comprising an A precursor        (which contains an X^(I) anion) and a first aqueous solvent with        a second aqueous solution comprising an A precursor (which        contains an X^(II) anion) and a second aqueous solvent (which        may be the same as or different to the first aqueous solvent) to        form an aqueous solution, and:        -   a) contacting the said aqueous solution with the said            organic solution; and        -   b) allowing a precipitate to form when the said aqueous and            organic solutions are contacted.

The skilled person will appreciate that by combining different Aprecursors and M precursors to create an aqueous solution and an organicsolution each containing a variety of ions, any compound of formula[A]_(a)[M]_(b)[X]_(c) may be created in the process of the invention bycontacting only one aqueous solution with only one organic solution.

Processing of Precipitate

The precipitate produced by the process of the invention comprises acompound of formula [A]_(a)[M]_(b)[X]_(c). The precipitate is itselftherefore a crystalline A/M/X material. However, the precipitate when itis initially formed is present in a reaction mixture comprising anaqueous solvent and an organic solvent. The reaction mixture may alsocomprise one or more A precursors, M precursors or X precursors, eitherin solution or as solid particles.

Typically, it is desirable to separate the solid precipitate from theliquid components of the reaction mixture. This may be referred to asrecovering the precipitate. Typically, therefore, the process comprisesrecovering the precipitate. For example, the process may comprise one ormore further steps to recover the precipitate.

In some embodiments, therefore, the process may further comprisefiltering the reaction mixture to obtain the solid precipitate. In otherembodiments, the process may comprise centrifuging the reaction mixtureand decanting most or all of the liquid from the centrifuged reactionmixture to obtain the solid precipitate. Alternatively the precipitatemay be allowed to settle without centrifugation before most or all ofthe liquid is decanted off the solid precipitate. A mixture of thesetechniques may be used to remove the liquid components of the reactionmixture from the solid precipitate.

If the precipitate is merely separated from the reaction mixture, thecrystalline A/M/X material therein may be contaminated with residues ofsolvent, with side-products, with solid particles of residual Aprecursor or M precursor or any other component. It is preferabletherefore to remove such contaminants from the precipitate before theprecipitate is used in any downstream application such as themanufacture of an optoelectronic device. Removal of contaminants fromthe precipitate may be referred to as purifying the precipitate.

An exemplary method of purifying the precipitate is to wash it. Washingof the precipitate involves exposing the precipitate to a solvent inwhich it is sparingly soluble or insoluble at the temperature of theprocess, and then removing the washing solvent from the precipitate(e.g. by decanting or filtration as described herein).

The precipitate may be washed before or after the precipitate isrecovered from the reaction mixture. For example, the reaction mixturemay be diluted with a solvent and subsequently the precipitate may berecovered. Alternatively or additionally, the precipitate may berecovered before it is washed.

Washing of the precipitate may be repeated multiple times, e.g. once,twice, three times, four times or five times.

In a preferred embodiment, the process of the invention comprises:

-   c) contacting an aqueous solution comprising an A precursor and an    aqueous solvent with an organic solution comprising an M precursor    and an organic solvent to form a reaction mixture;-   d) allowing a precipitate to form when the said aqueous and organic    solutions are contacted; and    -   recovering the precipitate from the reaction mixture and then        washing the precipitate.

Formation of a Thin Film

The precipitate formed by the process of the invention can be useddirectly in a variety of applications. However, in a particularlypreferred embodiment of the invention, the process involves optionallypurifying the precipitate and using the precipitate as a startingmaterial for the formation of a thin film comprising a crystalline A/M/Xmaterial. The inventors have surprisingly found that thin films ofcrystalline A/M/X material formed from a starting material comprising aprecipitate of the same crystalline A/M/X material have finely tunableproperties such as peak emission wavelength and emission intensity.

One method by which the precipitate can be used as a starting materialin the creation of a thin film is to dissolve the precipitate in anorganic solvent to form a film-forming solution, and then prepare a thinfilm from the film-forming solution.

Accordingly, the process of the invention is preferably a process forpreparing a thin film of said crystalline A/M/X material, the processfurther comprising:

-   c) optionally washing the precipitate;-   d) dissolving the precipitate in an organic solvent to form a    film-forming solution; and-   e) dispersing the film-forming solution on a substrate.

The organic solvent in which the precipitate is dissolved may be thesame as or different to the organic solvent present in the organicsolution containing an M precursor. Usually, however, these two organicsolvents are different.

The step of washing the precipitate is optional. It is possible toprepare a film-forming solution directly from the precipitate recoveredfrom the reaction mixture. However, washing removes traces of Aprecursor, M precursor and (where present) X precursor, as well assolvents. This may assist with ensuring precise control of thestoichiometry of the crystalline A/M/X material in the thin film. It istherefore preferable to wash the precipitate before preparing a thinfilm therefrom.

In a preferred embodiment, the process of the invention is a process forproducing a thin film of a crystalline A/M/X material, the processcomprising:

-   a) contacting an aqueous solution comprising an A precursor and an    aqueous solvent with an organic solution comprising an M precursor    and an organic solvent;-   b) allowing a precipitate to form when the said aqueous and organic    solutions are contacted;-   c) recovering and washing the precipitate;-   d) dissolving the precipitate in an organic solvent to form a    film-forming solution; and-   e) dispersing the film-forming solution on a substrate.

The film-forming solution may be dispersed, or disposed, on a substrateby any method. The film-forming solution may for instance be poured ordropped on the substrate. However, according to established techniquesfor formation of thin films it is convenient to spin-coat a substratewith a film-forming solution to promote thin film formation. Accordinglyit is preferred that step (e) comprises spin-coating the film-formingsolution on the substrate.

Once the film-forming solution has been disposed on a substrate, a thincrystalline film is allowed to form. This may be allowed to occurpassively. For example, the substrate and film-forming solution disposedor dispersed thereon may simply be left, in ambient conditions (e.g.room temperature). A thin crystal film will generally gradually form. Toobtain a thin film, though, it is preferable to remove the solvententirely from the film-forming solution, leaving the thin film in theabsence of solvent.

In a preferred embodiment, therefore, the process further comprises:

-   f) removing the organic solvent from the film-forming solution on    the substrate,

preferably by evaporation, preferably at a temperature of 15 to 150° C.,also preferably for at least 1 minute.

This process can be rather slow and it may be preferred to acceleratethe process by heating. Accordingly, in a preferred embodiment step (f)is performed at a temperature of at least 20° C., for example at least50° C. For example, the organic solvent may be removed in step (f) at atemperature of from 50° C. to 120° C.

Other methods of accelerating solvent loss and formation of a thin filminclude applying a reduced pressure, for instance a pressure of 0.5 to0.99 atm.

In another embodiment, rather than preparing a film-forming solutionfrom which a thin film can be created, the thin film may be created by avapour deposition process. In a further embodiment of the process of theinvention, therefore, the process is a process for preparing a thin filmof said crystalline A/M/X material, the process further comprising:

-   c′) optionally washing the precipitate;-   d′) vapourising the precipitate; and-   e′) depositing the vapourised precipitate on a substrate.

As was explained above, the step of washing the precipitate is optionalas it is possible to prepare a film-forming solution directly from theprecipitate recovered from the reaction mixture. However, washingremoves traces of A precursor, M precursor and (where present) Xprecursor, as well as solvents. This may assist with ensuring precisecontrol of the stoichiometry of the crystalline A/M/X material in thethin film. It is therefore preferable to wash the precipitate beforepreparing a thin film therefrom.

Consequently, in another preferred embodiment, the process of theinvention is a process for producing a thin film of a crystalline A/M/Xmaterial, the process comprising:

-   a) contacting an aqueous solution comprising an A precursor and an    aqueous solvent with an organic solution comprising an M precursor    and an organic solvent;-   b) allowing a precipitate to form when the said aqueous and organic    solutions are contacted;-   c′) recovering and washing the precipitate;-   d′) vapourising the precipitate; and-   e′) depositing the vapourised precipitate on a substrate.

Methods of vapourising the precipitate and disposing them on a substrateare known in the art. Typically, the process comprises heating anevacuated chamber comprising the optionally washed precipitate, to forma vapour, and then allowing that vapour to come into contact with thesubstrate.

Compound of Formula [A]_(a)[M]_(b)[X]_(c)

The process of the invention produces a crystalline A/M/X materialcomprising a compound of formula [A]_(a)[M]_(b)[X]_(c). The compound offormula [A]_(a[)M]_(b)[X]_(c) is crystalline. Within this compound, [A]comprises one or more A cations; [M] comprises one or more M cations;[X] comprises one or more X anions; a is a number from 1 to 6; b is anumber from 1 to 6; and c is a number from 1 to 18. a is often a numberfrom 1 to 4, b is often a number from 1 to 3, and c is often a numberfrom 1 to 8.

Each of a, b and c may or may not be an integer. For instance, a, b or cmay not be an integer where the compound adopts a structure havingvacancies such that the crystal lattice is not completely filled. Themethod of the invention provides very good control over stoichiometry ofthe product and so is well-suited for forming structures where a, b or cis not an integer (for instance a structure having vacancies in one ormore of the A, M or X sites). Accordingly, in some embodiments, one ormore of a, b and c is a non-integer value. For example, one of a, b andc may be a non-integer value. In one embodiment, a is a non-integervalue. In another embodiment, b is a non-integer value. In yet anotherembodiment, c is a non-integer value.

In other embodiments, each of a, b and c are integer values. Thus, insome embodiments, a is an integer from 1 to 6; b is an integer from 1 to6; and c is an integer from 1 to 18. a is often an integer from 1 to 4,b is often an integer from 1 to 3, and c is often an integer from 1 to8.

In the compound of formula [A]_(a)[M]_(b)[X]_(c), generally:

-   [A] comprises one or more A cations, which A cations may for    instance be selected from alkali metal cations or organic    monocations;-   [M] comprises one or more M cations which are metal or metalloid    cations selected from Pd⁴⁺, W⁴⁺, Re⁴⁺, Os⁴⁺, Ir⁴⁺, Pt⁴⁺, Sn⁴⁺, Pb⁴⁺,    Ge⁴⁺, Te⁴⁺, Bi³⁺, Sb³⁺, Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Fe²⁺,    Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺, preferably Sn²⁺, Pb²⁺,    Cu²⁺, Ge²⁺, and Ni²⁺; particularly preferably Pb²⁺;-   [X] comprises one or more X anions selected from halide anions (e.g.    Cl⁻, Br⁻, and I⁻), O²⁻, S²⁻, Se²⁻, and Te²⁻;-   a is a number from 1 to 4;-   b is a number from 1 to 3; and-   c is a number from 1 to 8.

Preferably the compound of formula [A]_(a)[M]_(b)[X]_(c) comprises aperovskite. The compound of formula [A]_(a)[M]_(b)[X]_(c) oftencomprises a metal halide perovskite.

In general, said one or more A cations are monocations and said one ormore M cations are dications. However, other ionic charges are possible,as is apparent from the various exemplary embodiments of the compound offormula [A]_(a)[M]_(b)[X]_(c) discussed below.

Typically, a = 1, b = 1 and c = 3. Thus, typically, the crystallineA/M/X material comprises: a perovskite of formula (I):

wherein: [A] comprises one or more A cations which are monocations; [M]comprises one or more M cations which are metal or metalloid dications;and [X] comprises one or more anions which are halide anions.

[A] comprises one or more A cations which may be organic and/orinorganic monocations. Where an A species is an organic monocation, A istypically selected from C₁₋₁₀ alkylammonium, C₂₋₁₀ alkenylammonium,C₁₋₁₀ alkyliminium, C₃₋₁₀ cycloalkylammonium and C₃₋₁₀ cycloalkyliminiumoptionally substituted with one or more substituents selected fromamino, C₁₋₆ alkylamino, imino, C₁₋₆ alkylimino, C₁₋₆ alkyl, C₂₋₆alkenyl, C₃₋₆ cycloalkyl and C₆₋₁₂ aryl. For example, A may be selectedfrom (CH₃NH₃)⁺, (CH₃CH₂NH₃)⁺, (CH₃CH₂CH₂NH₃)⁺, (N(CH₃)₄)⁺,(H₂N—C(H)═NH₂)⁺ and (H₂N—C(CH₃)═NH₂)⁺. Where an A species is aninorganic monocation, A is typically an alkali metal monocation (thatis, a monocation of a metal found in Group 1 of the periodic table), forexample Cs⁺ or Rb⁺.

[A] usually comprises one, two or three A monocations. Preferably, Acomprises a single cation selected from (CH₃NH₃)⁺, (CH₃CH₂NH₃)⁺,(CH₃CH₂CH₂NH₃)⁺, (N(CH₃)₄)⁺, (H₂N—C(H)═NH₂)⁺, (H₂N—C(CH₃)═NH₂)⁺, Cs⁺ andRb⁺. Alternatively, [A] may comprise two cations selected from thisgroup, for instance Cs⁺ and (H₂N—C(CH₃)═NH₂)⁺, or Cs⁺ and(H₂N—C(H)═NH₂)⁺, or for instance Cs⁺ and Rb⁺.

[M] comprises one or more M cations which are metal or metalloiddications. For instance, [M] may comprise an M cation selected fromCa²⁺, Sr²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺,Yb²⁺ and Eu²⁺. [M] typically comprises one or two M cations, and theseare generally selected from Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Fe²⁺,Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺; preferably Pb²⁺,

[X] comprises one or more X anions. For instance, [X] may comprise ananion selected from F⁻, Br⁻, Cl⁻ and I⁻. [X] typically comprises one,two or three X anions and these are generally selected from Br⁻, Cl⁻ andI⁻. [X] may for instance consist of two X anions, such as Cl and Br, orBr and I, or Cl and I.

In one embodiment, the perovskite is a perovskite of the formula (IA):

wherein A^(I) and A^(II) are as defined above with respect to A, whereinM and X are as defined above and wherein x is greater than 0 and lessthan 1. In a preferred embodiment, A^(I) and A^(II) are each selectedfrom (CH₃NH₃)⁺, (CH₃CH₂NH₃)⁺, (CH₃CH₂CH₂NH₃)⁺, (N(CH₃)₄)⁺,(H₂N—C(H)═NH₂)⁺, (H₂N—C(CH₃)═NH₂)⁺, Cs⁺ and Rb⁺; M is Pb²⁺ and X isselected from Br⁻, Cl⁻ and I⁻. A^(I) and A^(II) may for instance be(H₂N—C(CH₃)═NH₂)⁺ and Cs⁺ respectively, or they may be Cs⁺ and(H₂N—C(H)═NH₂)⁺ respectively. Alternatively, they may be Cs⁺ and Rb⁺respectively.

In one embodiment, the perovskite is a perovskite compound of theformula (IB):

wherein A and M are as defined above, wherein X^(I) and X^(II) are asdefined above in relation to X and wherein y is greater than 0 and lessthan 1. In a preferred embodiment, A is selected from (CH₃NH₃)⁺,(CH₃CH₂NH₃)⁺, (CH₃CH₂CH₂NH₃)⁺, (N(CH₃)₄)⁺, (H₂N—C(H)═NH₂)⁺,(H₂N—C(CH₃)═NH₂)⁺, Cs⁺ and Rb⁺; M is Pb²⁺ and X^(I) and X^(II) are eachselected from Br⁻, Cl⁻ and I⁻.

In a particularly preferred embodiment, the perovskite is a perovskiteof the formula (IC):

wherein A^(I) and A^(II) are as defined above with respect to A, M is asdefined above, X^(I) and X^(II) are as defined above in relation to Xand wherein x and y are both greater than 0 and less than 1. In apreferred embodiment, A^(I) and A^(II) are each selected from (CH₃NH₃)⁺,(CH₃CH₂NH₃)⁺, (CH₃CH₂CH₂NH₃)⁺, (N(CH₃)₄)⁺, (H₂N—C(H)═NH₂)⁺,(H₂N—C(CH₃)═NH₂)⁺, Cs⁺ and Rb⁺; M is Pb² and X^(I) and X^(II) are eachselected from Br⁻, Cl⁻ and I⁻.

Generally, in the above, x is from 0.01 to 0.99. For example, x may befrom 0.05 to 0.95 or 0.1 to 0.9. Similarly, in the above, y is generallyfrom 0.01 to 0.99. For example, y may be from 0.05 to 0.95 or 0.1 to0.9. In an exemplary embodiment, x and y are both from 0.1 to 0.9.

In exemplary embodiments, the compound of formula [A]_(a)[M]_(b)[X]_(c)is selected from CsPbBr₃, CsPbCl₃, CsPbI₃, CsPb(Br_(y)Cl_(1-y))₃,CsPb(Br_(y)I_(1-y))₃, CsPb(Cl_(y)I_(1-y))₃, (Cs_(x)Rb_(1-x))PbBr₃,(Cs_(x)Rb_(1-x))PbCl₃, (Cs_(x)Rb_(1-x))PbI₃,(Cs_(x)Rb_(1-x))Pb(Br_(y)Cl_(1-y))₃, (Cs_(x)Rb_(1-x))Pb(Br_(y)I_(1-y))₃,and (Cs_(x)Rb₁₋ _(x))Pb(Cl_(y)I_(1-y))₃, wherein x and y are bothgreater than 0 and less than 1. Preferably, in these embodiments, x isfrom 0.01 to 0.99 and y is from 0.01 to 0.99; more preferably, x is from0.05 to 0.95 and y is from 0.05 to 0.95.

In one embodiment, a = 2, b = 1 and c = 4. In that embodiment, thecrystalline A/M/X material comprises a compound (a “2D layeredperovskite”) of formula (II):

wherein: [A] comprises one or more A cations which are monocations; [M]comprises one or more M cations which are metal or metalloid dications;and [X] comprises one or more X anions which are halide anions. In thisembodiment, the A and M cations, and the X anions, are as defined abovein relation to the perovskite of formula (I).

In another embodiment, a = 2, b = 1 and c = 6. In that embodiment, thecrystalline A/M/X material may in that case comprise a hexahalometallateof formula (III):

wherein: [A] comprises one or more A cations which are monocations; [M]comprises one or more M cations which are metal or metalloidtetracations; and [X] comprises one or more X anions which are halideanions.

The hexahalometallate of formula (III) may in a preferred embodiment bea mixed monocation hexahalometallate. In a mixed monocationhexahalometallate, [A] comprises at least two A cations which aremonocations; [M] comprises at least one M cation which is a metal ormetalloid tetracation (and typically [M] comprises a single M cationwhich is a metal or metalloid tetracation); and [X] comprises at leastone X anion which is a halide anion (and typically [X] comprises asingle halide anion or two types of halide anion). In a mixed metalhexahalometallate, [A] comprises at least one monocation (and typically[A] is a single monocation or two types of monocation); [M] comprises atleast two metal or metalloid tetracations (for instance Ge⁴⁺ and Sn⁴⁺);and [X] comprises at least one halide anion (and typically [X] is asingle halide anion or two types of halide anion). In a mixed halidehexahalometallate, [A] comprises at least one monocation (and typically[A] is a single monocation or two types of monocation); [M] comprises atleast one metal or metalloid tetracation (and typically [M] is a singlemetal tetra cation); and [X] comprises at least two halide anions, forinstance Br⁻ and Cl⁻ or Br⁻ and I⁻.

[A] may comprise at least one A monocation selected from any suitablemonocations, such as those described above for a perovskite. In the caseof a hexahalometallate, each A cation is typically selected from Li⁺,Na⁺, K⁺, Rb⁺, Cs⁺, NH₄ ⁺ and monovalent organic cations. Monovalentorganic cations are singly positively charged organic cations, whichmay, for instance, have a molecular weight of no greater than 500 g/mol.For instance, [A] may be a single A cation which is selected from Li⁺,Na⁺, K⁺, Rb⁺, Cs⁺, NH₄ ⁺ and monovalent organic cations. [A] preferablycomprises at least one A cation which is a monocation selected from Rb⁺,Cs⁺, NH₄ ⁺ and monovalent organic cations. For instance, [A] may be asingle inorganic A monocation selected from Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ andNH₄ ⁺. In another embodiment, [A] may be at least one monovalent organicA cation. For instance, [A] may be a single monovalent organic A cation.In one embodiment, [A] is (CH₃NH₃)⁺. In another embodiment, [A] is(H₂N—C(H)═NH₂)⁺.

Preferably, [A] comprises two or more types of A cation. [A] may be asingle A monocation, or indeed two A monocations, each of which isindependently selected from K⁺, Rb⁺, Cs⁺, NH₄ ⁺, (CH₃NH₃)⁺,(CH₃CH₂NH₃)⁺, (CH₃CH₂CH₂NH₃)⁺, (N(CH₃)₄)⁺, (N(CH₂CH₃)₄)⁺,(N(CH₂CH₂CH₃)₄)⁺, (H₂N—C(H)═NH₂)⁺ and (H₂N—C(CH₃)═NH₂)⁺.

[M] may comprise one or more M cations which are selected from suitablemetal or metalloid tetracations. Metals include elements of groups 3 to12 of the Periodic Table of the Elements and Ga, In, Tl, Sn, Pb, Bi andPo. Metalloids include Si, Ge, As, Sb, and Te. For instance, [M] maycomprise at least one M cation which is a metal or metalloid tetracationselected from Ti^(4 |), V^(4 |), Mn^(4 |), Fe^(4 |), Co^(4 |), Zr^(4 |),Nb^(4 |), Mo^(4 |), Ru^(4 |), Rh^(4 |), Pd^(4 |), Hf^(4 |), Ta^(4 |),W^(4 |), Re^(4 |), Os⁴⁺, Ir⁴⁺, Pt⁴⁺, Sn⁴⁺, Pb⁴⁺, Po⁴⁺, Si⁴⁺, Ge⁴⁺, andTc⁴⁺. Typically, [M] comprises at least one metal or metalloidtetracation selected from Pd⁴⁺, W⁴⁺, Re⁴⁺, Os⁴⁺, Ir⁴⁺, Pt⁴⁺, Sn⁴⁺, Pb⁴⁺,Ge⁴⁺, and Te⁴⁺. For instance, [M] may be a single metal or metalloidtetracation selected from Pd⁴⁺, W⁴⁺, Re⁴⁺, Os⁴⁺, Ir⁴⁺, Pt⁴⁺, Sn⁴⁺, Pb⁴⁺,Ge⁴⁺, and Te⁴⁺.

Typically, [M] comprises at least one M cation which is a metal ormetalloid tetracation selected from Sn⁴⁺, Te⁴⁺, Ge⁴⁺ and Re⁴⁺. In oneembodiment [M] comprises at least one M cation which is a metal ormetalloid tetracation selected from Pb⁴⁺, Sn⁴⁺, Te⁴⁺, Ge⁴⁺ and Re⁴⁺. Forinstance, [M] may comprise an M cation which is at least one metal ormetalloid tetracation selected from Pb^(4 |), Sn^(4 |), Te^(4 |) andGe^(4 |) . Preferably, [M] comprises at least one metal or metalloidtetracation selected from Sn⁴⁺, Te⁴⁺, and Ge⁴⁺. As discussed above, thehexahalometallate compound may be a mixed-metal or a single-metalhexahalometallate. Preferably, the hexahalometallate compound is asingle-metal hexahalometallate compound. More preferably, [M] is asingle metal or metalloid tetracation selected from Sn⁴⁺, Te⁴⁺, andGe⁴⁺. For instance, [M] may be a single metal or metalloid tetracationwhich is Te⁴⁺. For instance, [M] may be a single metal or metalloidtetracation which is Ge⁴⁺. Most preferably, [M] is a single metal ormetalloid tetracation which is Sn⁴⁺.

[X] may comprise at least one X anion which is a halide anion. [X]therefore comprises at least one halide anion selected from F⁻, Cl⁻, Br⁻and I⁻. Typically, [X] comprises at least one halide anion selected fromCl⁻, Br⁻ and I⁻. The hexahalometallate compound may be a mixed-halidehexahalometallate or a single-halide hexahalometallate. If thehexahalometallate is mixed, [X] comprises two, three or four halideanions selected from F⁻, Cl⁻, Br⁻ and I⁻. Typically, in a mixed-halidecompound, [X] comprises two halide anions selected from F⁻, Cl⁻, Br⁻ andI⁻.

In some embodiments, [A] is a single monocation and [M] is a singlemetal or metalloid tetracation. Thus, the crystalline material may, forinstance, comprise a hexahalometallate compound of formula (IIIA)

wherein: A is a monocation; M is a metal or metalloid tetracation; and[X] is at least one halide anion. [X] may be one, two or three halideanions selected from F⁻, Cl⁻, Br⁻ and I⁻, and preferably selected fromCl⁻, Br⁻ and I⁻. In formula (IIIA), [X] is preferably one or two halideanions selected from Cl⁻, Br⁻ and I⁻.

The crystalline material may, for instance, comprise, or consistessentially of, a hexahalometallate compound of formula (IIIB)

wherein: A is a monocation (i.e. the second cation); M is a metal ormetalloid tetracation (i.e. the first cation); X and X′ are eachindependently a (different) halide anion (i.e. two second anions); and yis from 0 to 6. When y is 0 or 6, the hexahalometallate compound is asingle-halide compound. When y is from 0.01 to 5.99 the compound is amixed-halidehexahalometallate compound. When the compound is amixed-halide compound, y may be from 0.05 to 5.95. For instance, y maybe from 1.00 to 5.00.

The hexahalometallate compound may, for instance, be A₂SnF_(6-y)Cl_(y),A₂SnF_(6-y)Br_(y), A₂SnF_(6-y)I_(y), A₂SnCl_(6-y)Br_(y),A₂SnCl_(6-y)I_(y), A₂SnBr_(6-y)I_(y), A₂TeF_(6-y)Cl_(y),A₂TeF_(6-y)Br_(y), A₂TeF_(6-y)I_(y), A₂TeCl_(6-y)Br_(y),A₂TeCl_(6-y)I_(y), A₂TeBr_(6-y)I_(y), A₂GeF_(6-y)Cl_(y),A₂GeF_(6-y)Br_(y), A₂GeF_(6-y)I_(y), A₂GeCl_(6-y)Br_(y),A₂GeCl_(6-y)I_(y), A₂GeBr_(6-y)I_(y), A₂ReF_(6-y)Cl_(y),A₂ReF_(6-y)Br_(y), A₂ReF_(6-y)I_(y), A₂ReCl_(6-y)Br_(y),A₂ReCl_(6-y)I_(y) or A₂ReBr₆₋ _(y)I_(y), wherein: A is K⁺, Rb⁺, Cs⁺,(R¹NH₃)⁺, (NR² ₄)⁺, or (H₂N—C(R¹)═NH₂)⁺, wherein R¹ is H, a substitutedor unsubstituted C₁₋₂₀ alkyl group or a substituted or unsubstitutedaryl group, and R² is a substituted or unsubstituted C₁₋₁₀ alkyl group;and y is from 0 to 6. Optionally, y is from 0.01 to 5.99. If thehexahalometallate compound is a mixed-halide compound, y is typicallyfrom 1.00 to 5.00. A may be as defined above. For instance, A may beCs⁺, NH₄ ⁺, (CH₃NH₃)⁺, (CH₃CH₂NH₃)⁺, (N(CH₃)₄)⁺, (N(CH₂CH₃)₄)⁺,(H₂N—C(H)═NH₂)⁺ or (H₂N—C(CH₃)═NH₂)⁺, for instance Cs⁺, NH₄ ⁺, or(CH₃NH₃)⁺.

The hexahalometallate compound may typically be A₂SnF_(6-y)Cl_(y),A₂SnF_(6-y)Br_(y), A₂SnF_(6-y)I_(y), A₂SnCl_(6-y)Br_(y),A₂SnCl_(6-y)I_(y), or A₂SnBr_(6-y)I_(y), wherein: A is K⁺, Rb⁺, Cs⁺,(R¹NH₃)⁺, (NR² ₄)⁺, or (H₂N—C(R¹)═NH₂)⁺, or A is as defined herein,wherein R¹ is H, a substituted or unsubstituted C₁₋₂₀ alkyl group or asubstituted or unsubstituted aryl group, or R² is a substituted orunsubstituted C₁₋₁₀ alkyl group; and y is from 0 to 6.

In another embodiment, the hexahalometallate compound isA₂GeF_(6-y)Cl_(y), A₂GeF_(6-y)Br_(y), A₂GeF_(6-y)I_(y),A₂GeCl_(6-y)Br_(y), A₂GeCl_(6-y)I_(y), or A₂GeBr_(6-y)I_(y), wherein: Ais K⁺, Rb⁺, Cs⁺, (R¹NH₃)⁺, (NR² ₄)⁺, or (H₂N—C(R¹)═NH₂)⁺, or A is asdefined herein, wherein R¹ is H, a substituted or unsubstituted C₁₋₂₀alkyl group or a substituted or unsubstituted aryl group, and R² is asubstituted or unsubstituted C₁₋₁₀ alkyl group; and y is from 0 to 6.

The hexahalometallate compound may, for instance, be A₂TeF_(6-y)Cl_(y),A₂TeF_(6-y)Br_(y), A₂TeF_(6-y)I_(y), A₂TeCl_(6-y)Br_(y),A₂TeCl_(6-y)I_(y), or A₂TeBr_(6-y)I_(y), wherein: A is K⁺, Rb⁺, Cs⁺,(R¹NH₃)⁺, (NR² ₄)⁺, or (H₂N—C(R¹)═NH₂)⁺, or A is as defined herein,wherein R¹ is H, a substituted or unsubstituted C₁₋₂₀ alkyl group or asubstituted or unsubstituted aryl group, and R² is a substituted orunsubstituted C₁₋₁₀ alkyl group; and y is from 0 to 6 or y is as definedherein.

Often, y will be from 1.50 to 2.50. For instance, y may be from 1.80 to2.20. This may occur if the compound is produced using two equivalentsof AX′ and one equivalent of MX₄, as discussed below.

In some embodiments, all of the ions are single anions. Thus, thecrystalline material may comprise, or consist essentially of, ahexahalometallate compound of formula (IIIC)

wherein: A is a monocation; M is a metal or metalloid tetracation; and Xis a halide anion. A, M and X may be as defined herein.

The hexahalometallate compound may be A₂SnF₆, A₂SnCl₆, A₂SnBr₆, A₂SnI₆,A₂TeF₆, A₂TeCl₆, A₂TeBr₆, A₂TeI₆, A₂GeF₆, A₂GeCl₆, A₂GeBr₆, A₂GeI₆,A₂ReF₆, A₂ReCl₆, A₂ReBr₆ or A₂ReI₆, wherein: A is K⁺, Rb⁺, Cs⁺,(R¹NH₃)⁺, (NR² ₄)⁺, or (H₂N—C(R¹)═NH₂)⁺, wherein R¹ is H, a substitutedor unsubstituted C₁₋₂₀ alkyl group or a substituted or unsubstitutedaryl group, and R² is a substituted or unsubstituted C₁₋₁₀ alkyl group.A may be as defined herein. Preferably, the hexahalometallate compoundis Cs₂SnI₆, Cs₂SnBr₆, Cs₂SnBr_(6-y)I_(y), Cs₂SnCl₆₋ _(y)I_(y),Cs₂SnCl_(6-y)Br_(y), (CH₃NH₃)₂SnI₆, (CH₃NH₃)₂SnBr₆,(CH₃NH₃)₂SnBr_(6-y)I_(y), (CH₃NH₃)₂SnCl₆₋ _(y)I_(y),(CH₃NH₃)₂SnCl_(6-y)Br_(y), (H₂N—C(H)═NH₂)₂SnI₆, (H₂N—C(H)═NH₂)₂SnBr₆,(H₂N-C(H)=NH₂)₂SnBr_(6-y)I_(y), (H₂N-C(H)=NH₂)₂SnCl_(6-y)I_(y) or(H₂N-C(H)=NH₂)₂SnCl_(6-y)Br_(y) wherein y is from 0.01 to 5.99. Forexample, the hexahalometallate compound may be (CH₃NH₃)₂SnI₆,(CH₃NH₃)₂SnBr₆, (CH₃NH₃)₂SnCl₆, (H₂N—C(H)═NH₂)₂SnI₆,(H₂N—C(H)═NH₂)₂SnBr₆ or (H₂N—C(H)═NH₂)₂SnCl₆. The hexahalometallatecompound may be Cs₂SnI₆, Cs₂SnBr₆, Cs₂SnCl_(6-y)Br_(y), (CH₃NH₃)₂SnI₆,(CH₃NH₃)₂SnBr₆, or (H₂N—C(H)═NH₂)₂SnI₆.

The crystalline A/M/X material may comprise a bismuth or antimonyhalogenometallate. For instance, the crystalline A/M/X material maycomprise a halogenometallate compound comprising: (i) one or moremonocations ([A]) or one or more dications ([B]); (ii) one or more metalor metalloid trications ([M]); and (iii) one or more halide anions([X]). The compound may be a compound of formula BBiXs, B₂BiX₇ or B₃BiX₉where B is (H₃NCH₂NH₃)²⁺, (H₃N(CH₂)₂NH₃)²⁺, (H₃N(CH₂)₃NH₃)²⁺,(H₃N(CH₂)₄NH₃)²⁺, (H₃N(CH₂)₅NH₃)²⁺, (H₃N(CH₂)₆NH₃)²⁺, (H₃N(CH₂)₇NH₃)²⁺,(H₃N(CH₂)₈NH₃)²⁺ or (H₃N-C₆H₄-NH₃)²⁺ and X is I⁻, Br⁻ or Cl⁻, preferablyI⁻.

In yet further embodiments, the crystalline A/M/X materials of theinvention may be double perovskites. Such compounds are defined in WO2017/037448, the entire contents of which is incorporated herein byreference. Typically, the compound is a double perovskite compound offormula (IV):

wherein: [A] comprises one or more A cations which are monocations, asdefined herein; [B⁺] and [B³⁺] are equivalent to [M] where M comprisesone or more M cations which are monocations and one or more M cationswhich are trications; and [X] comprises one or more X anions which arehalide anions.

The one or more M cations which are monocations comprised in [B⁺] aretypically selected from metal and metalloid monocations. Preferably, theone or more M cations which are monocations are selected from Li⁺, Na⁺,K⁺, Rb⁺, Cs⁺, Cu⁺, Ag⁺, Au⁺ and Hg⁺. More preferably, the one or more Mcations which are monocations are selected from Cu⁺, Ag⁺ and Au⁺. Mostpreferably, the one or more M cations which are monocations are selectedfrom Ag⁺ and Au⁺. For instance, [B⁺] may be one monocation which is Ag⁺or [B⁺] may be one monocation which is Au⁺.

The one or more M cations which are trications comprised in [B³⁺] aretypically selected from metal and metalloid trications. Preferably, theone or more M cations which are trications are selected from Bi³⁺, Sb³⁺,Cr³⁺, Fe³⁺, Co³⁺, Ga³⁺, As³⁺, Ru³⁺, Rh³⁺, In³⁺, Ir³⁺ and Au³⁺. Morepreferably, the one or more M cations which are trications are selectedfrom Bi³⁺ and Sb³⁺. For instance, [B³⁺] may be one trication which isBi³⁺ or [B³⁺] may be one trication which is Sb³⁺. Bismuth has relativelylow toxicity compared with heavy metals such as lead. In someembodiments, the one or more M cations which are monocations (in [B⁺])are selected from Cu ^(|), Ag ^(|) and Au ^(|) and the one or more Mcations which are trications (in [B³ ^(|])) are selected from Bi³⁺ andSb³⁺.

An exemplary double perovskite is Cs₂BiAgBr₆.

Typically, where the compound is a double perovskite it is a compound offormula (IVa):

wherein: the A cation is as defined herein; B⁺ is an M cation which is amonocation as defined herein; B³⁺ is an M cation which is a trication asdefined herein; and [X] comprises one or more X anions which are halideanions, for instance two or more halide anions, preferably a singlehalide anion.

In yet another embodiment, the compound may be a layered doubleperovskite compound of formula (V):

wherein: [A], [B⁺], [B³⁺] and [X] are as defined above. In someembodiments, the layered double perovskite compound is a doubleperovskite compound of formula (Va):

wherein: the A cation is as defined herein; B⁺ is an M cation which is amonocation as defined herein; B³⁺ is an M cation which is a trication asdefined herein; and [X] comprises one or more X anions which are halideanions, for instance two or more halide anions, preferably a singlehalide anion or two kinds of halide anion.

In yet another embodiment, the compound may be a compound of formula(VI):

wherein: [A], [M] and [X] are as defined above (in relation to, forinstance, compounds of formula (I) or (II)). However, preferably thecompound is not a compound of formula (VI). Where the compound is acompound of formula (VI), the compound may preferably be a compound offormula (VIA)

that is, a compound wherein [A] comprises two types of A monoacation. Inother preferred embodiments, the compound of formula (VI) may be acompound of formula (VIB):

that is, a compound of formula (VI) wherein [X] comprises two types of Xanion. In yet other preferred embodiments, the compound of formula (VI)may be a compound of formula (VIC):

that is, a compound of formula (VI) wherein [A] comprises two types of Amonoacation and [X] comprises two types of X anion. In formulae (VIa),(VIb) and (VIc), each of: [A], [M] and [X] are as defined above (inrelation to, for instance, compounds of formula (I) or (II)).

In another embodiment, a = 1, b = 1 and c = 4. In that embodiment, thecrystalline A/M/X material may in that case comprise a compound offormula (VII):

wherein: [A] comprises one or more A cations which are monocations; [M]comprises one or more M cations which are metal or metalloid trications;and [X] comprises one or more X anions which are halide anions. The Amonocations and M trications are as defined herein. An exemplarycompound of formula (VII) is AgBiI₄.

It should be understood that the invention also encompasses processesfor producing variants of the above-described structures (I), (II),(III), (IV), (V), (VI) and (VII) where one or more of the relevant a, band c values are non-integer values.

Preferably, the compound of formula [A]_(a)[M]_(b)[X]_(c) is a compoundof formula [A][M][X]₃, a compound of formula [A]₄[M][X]₆ or a compoundof formula [A]_(2[)M][X]₆. For example, in preferred embodiments thecompound of formula [A]_(a)[M]_(h)[X]_(c) is a compound of formula is acompound of formula (IA), (IB), (IC), (IIIA), (IIIB),(IIIC), (VIA),(VIB), or (VIC).

In some embodiments, the compound of formula [A]_(a)[M]_(b)[X]_(c) is acompound wherein [A] comprises two or more different A cations. Forexamples, [A] may contain two types of cation. In some embodiments, thecompound of formula [A]_(a)[M]_(d)[X]_(c) is a compound wherein [X]comprises two or more different X anions. For example, [X] may containtwo types of anion, e.g. halide anions. In one aspect of each of theseembodiments, the compound of formula [A]_(a)[M]_(b)[X]_(c) is a compoundwherein [A] comprises two or more different A cations and wherein [X]comprises two or more different X anions. For example, [A] may containtwo types of A cation and two types of X anion (e.g. two types of halideanion). The process of the invention is particularly useful forpreparing compounds and materials having mixed cation or mixed halidesites as the stoichiometry can be more carefully controlled than ispossible with conventional methods utilising a single solvent.

In one preferred embodiment, [A] comprises Cs⁺ and Rb⁺ _(;) forinstance, [A] may consist of Cs⁺ and Rb⁺. In another preferredembodiment, [X] comprises two halide anions, preferably Br- and Cl⁻; forexample, [X] may consist of Br⁻ and Cl⁻. In a preferred aspect of theseembodiments, [A] comprises Cs⁺ and Rb⁺ and [X] comprises two halideanions, preferably Br⁻ and Cl⁻. In a particularly preferred example, [A]consists of Cs⁺ and Rb⁺ and [X] consists of two halide anions, e.g. Br⁻and Cl⁻. In each embodiment in this paragraph, the compound of formula[A]_(a)[M]_(b)[X]_(c) is preferably a compound of formula [A][M][X]₃;[A]_(2[)M][X]₆ and [A]₄[M][X]₆.

Typically, each A cation is selected from an alkali metal cation, C₁₋₁₀alkylamammonium, C₂₋10 alkenylammonium, C₁₋₁₀ alkyliminium, C₃₋₁₀cycloalkylamammonium and C₃₋₁₀ cycloalkyliminium optionally substitutedwith one or more substituents selected from amine, C₁₋₆ alkylamine,imine, C₁₋₆ alkylimine, C₁₋₆ alkyl, C₂₋₆ alkenyl,, C₃₋₆ cycloalkyl andC₆₋₁₂ aryl. Preferably Each A cation is selected from Cs⁺, Rb⁺,methylammonium, ethylammonium, propylammonium. butylammonium,pentylammoium, hexylammonium, septylammonium, octylammonium, andguanidinium. Typically, [A] comprises one, two or three A cations;preferably [A] is one A cation. Preferably, each A cation is amonocation.

Generally each metal or metalloid M cation is selected from Li⁺, Na⁺,K⁺, Rb⁺, Cs⁺, Cu⁺, Ag⁺, Au ^(|), Hg ^(|), Ca^(2 |), Sr^(2 |), Cd^(2 |)Cu^(2 |) Ni^(2 |), Mn^(2 |), Fe² ^(|) Co^(2 |), Pd^(2 |) Ge^(2 |),Sn^(2 |), Pb^(2 |), Yb^(2 |), Eu^(2 |), Bi³⁺, Sb³⁺, Cr³⁺, Fc³⁺, Co³⁺,Ga³⁺, As³⁺, Ru³⁺, Rh³⁺, In³⁺, Ir³⁺ and Au³⁺. Usually, each metal ormetalloid M cation is selected from Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺,Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺, preferably Sn²⁺,Pb²⁺, Cu²⁺, Ge²⁺, and Ni²⁺; particularly preferably Pb²⁺.

Usually each X anion is selected from F⁻, Cl⁻, Br⁻ or I⁻, preferably Cl⁻or Br⁻.

A and M Precursors

An A precursor is a compound comprising one or more A cations present in[A]. Where [A] comprises one type of A cation, usually one A precursoris utilised in the process of the invention and that A precursor willgenerally consist of that A cation together with one or morecounterions. Where [A] comprises two or more types of A cation, one ormore A precursors may be utilised in the process of the invention. EachA precursor may comprise one or more of the two or more A cationspresent in [A]. Generally, each A precursor comprises one or two Acations. Preferably, each A precursor comprises one type of A cationbecause such precursors are generally commercially available in highpurity.

An A precursor is capable of reacting with one or more M precursors andoptionally also with one or more additional A precursors and one or moreX precursors to produce a crystalline A/M/X material as defined herein.

Preferably, the or each A precursor is a halide salt of the A cation orone of the A cations. That is, preferably the or each A precursor is acompound comprising:

-   (i) the A cation present in [A] or, where [A] comprises two or more    A cations, one or more of the A cations present in [A]; and-   (ii) one or more halide anions, preferably selected from Cl⁻, Br⁻    and I⁻.

Typically, the or each A precursor comprises one or two halide anions,preferably one halide anion.

In an exemplary embodiment, therefore, each A precursor contains one ortwo A cations as defined herein and one or two halide anions preferablyselected from Cl⁻, Br and I⁻; in a particularly preferred embodiment,each A precursor contains one A cation as defined herein and one halideanion preferably selected from Cl⁻, Br⁻ and I⁻.

An M precursor is a compound comprising one or more M cations present in[M]. Where [M] comprises one type of M cation, usually one M precursoris utilised in the process of the invention and that M precursor willgenerally consist of that M cation together with one or morecounterions. Where [M] comprises two or more types of M cation, one ormore M precursors may be utilised in the process of the invention. EachM precursor may comprise one or more of the two or more A cationspresent in [M]. Generally, each M precursor comprises one or two Mcations. Preferably, each M precursor comprises one type of M cationbecause such precursors are generally commercially available in highpurity.

An M precursor is capable of reacting with one or more A precursors andoptionally also with one or more additional M precursors and one or moreX precursors to produce a crystalline A/M/X material as defined herein.

Preferably, the or each M precursor is a halide salt of the M cation orone of the M cations. That is, preferably the or each M precursor is acompound comprising:

-   (i) the M cation present in [M] or, where [M] comprises two or more    M cations, one or more of the M cations present in [M]; and-   (ii) one or more halide anions, preferably selected from Cl⁻, Br⁻    and I⁻.

Typically, the or each M precursor comprises one or two halide anions,preferably one halide anion.

In an exemplary embodiment, therefore, each M precursor contains one ortwo M cations as defined herein and one or two halide anions preferablyselected from Cl⁻, Br⁻ and I⁻; in a particularly preferred embodiment,each M precursor contains one M cation as defined herein and one halideanion preferably selected from Cl⁻, Br⁻ and I⁻. In exemplaryembodiments, the M precursor may be selected from PbCl₂, PbBr₂ and PbI₂.

Aqueous and Organic Solvents and Solutions

The process of the invention involves one or more aqueous solutions. The(or each) aqueous solution comprises an aqueous solvent and an Aprecursor. Where multiple aqueous solutions are employed, the aqueoussolvent present in each aqueous solution may be the same or different.

Typically, the or each aqueous solution consists essentially of orconsists only of the A precursor and the aqueous solvent. However, otherspecies such as pH adjusting agents, stabilisers and so on may bepresent.

The aqueous solvent usually comprises less than 20% by volume of organicsolvents, and preferably comprises less than 10% by volume of organicsolvents. The aqueous solution usually comprises at least 80% by volumeof water, usually at least 90% by volume of water. Preferably, theaqueous solvent consists essentially of water. In some embodiments, theaqueous solvent consists entirely of water.

The process of the invention involves one or more organic solutions. The(or each) organic solution comprises an organic solvent and an Mprecursor. Where multiple organic solutions are employed, the organicsolvent present in each organic solution may be the same or different.

Typically, the or each organic solution consists essentially of orconsists only of: the M precursor, the organic solvent, and optionallyone or more X precursors (preferably one or more hydrohalic acids).However, other species such as pH adjusting agents, stabilisers and soon may be present.

The organic solution may comprise one or more organic solvents. Usually,the organic solution contains only one organic solvent.

The organic solvent in the organic solution comprises less than 20% byvolume of water, preferably less than 10% by volume of water. Typically,the organic solvent in said solution comprises a polar organic solvent.For instance, the said organic solvent may consist essentially of apolar organic solvent. Preferably the organic solvent comprises anorganic solvent that is miscible with water, for instance an organicsolvent that is miscible with water in all proportions. A particularlypreferred example of an organic solvent suitable for the organicsolution is dimethylformamide (DMF).

In some embodiments, the process of the invention utilises a furtherorganic solvent to dissolve the precipitate produced in the process ofthe invention, in order to create a film-forming solution. That organicsolvent may be the same or different to any of the solvents present inany organic solutions mentioned in step (a) of the process. The organicsolvent used to dissolve the precipitate and create a film-formingsolution may be referred to as “the organic solvent of step (d)”. Theorganic solvent of step (d) may contain a single organic solvent or maybe a mixture of organic solvents. The organic solvent of step (d)comprises a polar solvent, preferably DMSO and/or DMF. Where the organicsolvent of step (d) comprises a mixture of two or more solvents,preferably each organic solvent is a polar solvent.

Crystalline A/M/X Material

The process of the invention produces a crystalline A/M/X material whichcomprises one or more compounds of formula [A]a[M]_(b)[X]_(c). In someembodiments, the crystalline A/M/X material comprises two or morecompounds of formula [A]a[M]_(b)[X]_(c). For instance, the crystallineA/M/X material may comprise one, two, three, four or five compounds offormula [A]a[M]_(b)[X]_(c). Typically, the crystalline A/M/X materialcontains one, two or three compounds of formula [A]a[M]_(b)[X]_(c).Preferably, the crystalline A/M/X material contains one compound offormula [A]a[M]_(b)[X]_(c).

In one aspect, the invention provides a crystalline A/M/X materialobtained or obtainable by the process of the invention. The crystallineA/M/X may be in powder form, for instance as a precipitate obtaineddirectly from the process of the invention.

In a preferred embodiment, the said material is a “blue emitter”. Bythis is meant that the compound of formula [A]_(a[)M]_(h[)X]_(c) (or oneof the compounds of formula [A]_(a)[M]_(h)[X]_(c) in the material) iscapable of emitting blue light after stimulation by electrical energy orirradiation with blue, violet or ultraviolet light (e.g. light having awavelength of 500 nm or less). In this embodiment, that compound offormula [A]_(a)[M]_(b)[X]_(c) is capable of emitting light having awavelength of from 450 to 500 nm, e.g. from 455 to 495 nm, preferablyfrom 460 to 490 nm.

In another aspect, the invention provides a thin film obtainable orobtained by a process as described herein.

The thin film of the invention comprises a crystalline A/M/X material asdefined herein. The thin film typically comprises a crystalline orpolycrystalline layer, preferably a polycrystalline layer.

The thin film typically consists essentially of, or consists entirelyof, the crystalline A/M/X material as defined herein.

The thin film is typically a layer having a thickness of approximatelyfrom 1 nm to 10 µm. Usually, the thin film is a layer having a thicknessof from 1 nm to 5 µm. The thin film normally has a thickness of at least1 nm, for instance at least 10 nm or at least 50 nm, or for example athickness of greater than or equal to 100 nm. The thin film normally hasa thickness of less than 10 µm, e.g. 1 µm or less. Preferably, the layerof said photoactive material has a thickness of from 100 nm to 700 nm,for instance from 200 nm to 500 nm.

Photoactive Material

The crystalline A/M/X material is typically a semiconductor, and istypically a strong emitter and a stable emitter of visible light (i.e.light having a wavelength of from about 450 nm to about 700 nm).Accordingly, the crystalline A/M/X material is highly useful in a widevariety of electronic devices. Exemplary optoelectronic devices andtheir manufacture are described in WO 2013/171517, WO 2013/171518, WO2013/171520, WO 2014/045021, WO 2017/017441, WO 2017/037448 and WO2017/089819, the entire contents of which are incorporated herein byreference.

For incorporation into an optoelectronic device, the crystalline A/M/Xmaterial is typically provided as a photoactive material. Thephotoactive material may consist entirely of a A/M/X material as definedherein, or may comprise other components. Accordingly, the inventionprovides a photoactive material comprising the crystalline A/M/Xmaterial, for example a luminescent material comprising the crystallineA/M/X material.

The term “photoactive material”, as used herein, refers to a materialwhich can absorb and/or emit photons. A photoactive material can do oneor more of the following.

-   (i) Absorb photons, which may then generate free charge carriers    e.g. electrons and holes. These materials are referred to as    photoabsorbent materials.-   (ii) Absorb photons at energies higher than its band gap and rc-cmit    photons at energies of the band gap (these are referred to as    photoemissive materials). One type of photoemissive material is a    luminescent material, which is a material that emits light following    the absorption of photons, i.e. a phosphorescent or a fluorescent    material.-   (iii) Accept charge, both electrons and holes, which may    subsequently recombine and emit light.

Photoactive materials are examples of semiconducting materials. Asemiconductor or semiconducting material as used herein refers to amaterial with electrical conductivity intermediate in magnitude betweenthat of a conductor and a dielectric. A semiconductor may have a bandgap of from 0.5 to 3.5 eV, for instance from 0.5 to 2.5 eV or from 1.0to 2.0 eV (when measured at 300 K). The invention therefore provides asemiconducting material comprising the crystalline A/M/X materialdescribed herein.

The photoactive material of the invention is generally capable ofabsorbing and/or emitting photons in the visible region of the spectrum,for example in the blue region of the visible spectrum. The photoactivematerial may therefore be described as a photoemissive material (i.e. amaterial that can emit light) or a photoabsorbent material (i.e. amaterial that can absorb light). For instance, the photoactive materialof the invention can typically emit and/or absorb photons of at leastone wavelength of 450 to 700 nm, for example of 450 to 650 nm. In apreferred embodiment, the crystalline A/M/X materials have a peakemission wavelength in the range 450 to 500 nm, preferably 455 to 495nm, particularly preferably 460 to 490 nm. However, photoactivematerials which can absorb and emit photons typically do not show peakabsorbance and peak emission at exactly the same wavelength.

By peak absorbance is meant the wavelength at which the photoactivematerial absorbs photons most efficiently, and by peak emission is meantthe wavelength at which the photoactive material emits photons mostefficiently. Typically, the peak emission of the photoactive material is700 nm or less, for example 650 nm or less. In a preferred embodiment,the crystalline A/M/X materials of the invention have a peak emissionwavelength in the range 450 to 500 nm, preferably 455 to 495 nm,particularly preferably 460 to 490 nm.

The photoactive material may comprise greater than or equal to 5 wt% ofthe crystalline A/M/X material. Typically at least 50% of the weight ofthe photoactive materials consists of a crystalline A/M/X material asdefined herein. The photoactive material may comprise additionalcomponents as discussed herein, for example scaffold materials, matrixmaterials or coatings. Typically, though, the photoactive materialcomprises greater than or equal to 80 wt% of a crystalline A/M/Xmaterial as defined herein. Preferably, the photoactive materialcomprises greater than or equal to 95 wt% of a crystalline A/M/Xmaterial, for instance greater than or equal to 99 wt% of a crystallineA/M/X material. The photoactive material may consist, or consistessentially, of a crystalline A/M/X material.

The photoactive material is typically solid.

The photoactive material may comprise a crystalline A/M/X material asdefined herein. Accordingly, in one embodiment the photoactive materialcomprises a powder of the crystalline A/M/X material. In anotherembodiment, the photoactive material may comprise a thin film of thecrystalline A/M/X material. Often, the crystalline A/M/X material ispolycrystalline and accordingly the photoactive material thereforecomprises a polycrystalline A/M/X material of the invention. In otherembodiments, the photoactive material comprises nanocrystals of thecrystalline A/M/X material.

The photoactive material may be in any form. Typically the photoactivematerial is in the form of a layer. The photoactive material in the formof a layer is typically at least 1 nm thick. The layer of photoactivematerial may be up to 10 mm thick, for instance where the layer isintended to be a free-standing component of a device. Usually, the layerof photoactive material is from 2 nm to 1 mm thick, more usually from 5nm to 5 µm thick.

The layer of photoactive material may comprise a crystalline A/M/Xmaterial within the said layer. The amount of said material within thesaid layer may vary, depending upon other components in the layer suchas any coatings, or matrix or scaffold materials. In some embodiments,the photoactive material may consist essentially of a layer of acrystalline A/M/X material as defined herein. For example, thephotoactive material may consist entirely of a crystalline A/M/Xmaterial. However, more typically, the photoactive material may compriseat least 50% of a crystalline A/M/X material by weight, for example atleast 70, 80 or 80% of a crystalline A/M/X material by weight. In someembodiments, the photoactive material may comprise at least 95%, of acrystalline A/M/X material by weight. Typically, a layer of photoactivematerial according to the invention comprises up to 99.9% of acrystalline A/M/X material by weight.

The photoactive material may comprise a plurality of layers. Some or allof such layers may comprise a crystalline A/M/X material.

Where a photoactive material is in the form of a layer, a crystallineA/M/X material may be distributed evenly or unevenly throughout thelayer. For instance, the photoactive material may comprise a layerconsisting essentially of, or consisting only of, a crystalline A/M/Xmaterial. Alternatively or additionally, the photoactive material maycomprise a substrate having a crystalline A/M/X material on the saidsubstrate (for instance in powder form or thin film form).

In preferred embodiments, the photoactive material may comprise one ormore crystalline A/M/X material(s) of the invention in the form of athin film. A thin film typically comprises a polycrystalline materialdisposed on a substrate.

The photoactive material of the invention may comprise one or morecrystalline A/M/X material(s) together with one or more of each of asurface layer or coating.

In some embodiments, for example, the photoactive material of theinvention may comprise one or more crystalline A/M/X material(s)together with a passivating agent. Passivating agents are described inpublished application WO 2015/092397, the entire contents of which isincorporated herein by reference. In these embodiments, the photoactivematerial comprises a crystalline A/M/X material according to theinvention, and a passivating agent. The passivating agent is an organicspecies which can form halogen bonds or chalcogen-metal bonds tounder-coordinated moieties in the metal halide perovskite structure,which can lead to a self-assembled layer on a surface of a crystal orcrystallite of a crystalline A/M/X material.

Exemplary passivating agents are pyrrolidine, piperidine, morpholine,2H-pyrrole, 2-pyrroline, 3-pyrroline, pyrrole pyridine, napthelene,anthracene, phenanthrene, pyrene, fluoranthene, thiophene,3-hcxylthiophcnc or tetrahydrothiophene and iodopentafluorobenzene.Thus, in one embodiment, the photoactive material comprises one or morecrystalline A/M/X materials together with a passivating agent selectedfrom one or more of pyrrolidine, piperidine, morpholine, 2H-pyrrole,2-pyrroline, 3-pyrroline, pyrrole pyridine, napthelene, anthracene,phenanthrene, pyrene, fluoranthene, thiophene, 3-hexylthiophene ortetrahydrothiophene and iodopentafluorobenzene. The passivating agent istypically present on or at the surface of the crystalline material, andmay therefore be described as a surface species.

In some embodiments, the photoactive material may comprise a coating.Typically, the coating may be a protective coating which protects thecrystalline A/M/X material(s) in the photoactive material fromenvironmental factors such as moisture and oxygen.

A coating is compatible with the powder and with the thin film obtainedor obtainable by the process of the invention. For instance, a powdermay be deposited onto or into a coating; a coating may be present in theform of a layer over a thin film, or may encompass a thin film.

Suitable coatings include transparent polymers such as polyethylene(PE), poly (methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate(PC), polyvinyl chloride (PVC), poly (vinylidene fluoride), (PVDF),polyurene (PU), polyvinyl alcohol (PVA), cellulose acetate (CA),acrylonitrile-butadiene-styrene (ABS), polyimide (PEI),Polydimethylsiloxane. Preferred polymers are polyethylene orpolymethylmethacrylate (PMMA). Other suitable coatings includesilicones.

In a particularly preferred embodiment, the coating comprises an oxideor a metal or metalloid cation. Preferably, the coating comprises anoxide of a metal or metalloid cation, which oxide has a band gap of 4 eVor more. In a particularly preferred embodiment, the coating comprisesan oxide of one or more of Al, Si, Zr, Ga, Mg, Y, Ti, Ni and Zn,preferably Al or Si, most preferably Al.

In a preferred aspect of the invention, the photoactive material of theinvention comprises a thin film of a crystalline A/M/X material asdescribed herein comprising a coating on all or part of the said thinfilm. Particularly preferably, the coating comprises alumina and/orsilica; particularly preferably alumina (Al₂O₃).

The photoactive material may comprise a plurality of coatings. A coatingcomprised in a photoactive material may or may not be directly incontact with a crystalline A/M/X material.

In some embodiments, the photoactive material of the invention comprisesa matrix material. That is, the photoactive material of the inventionmay comprise one or more crystalline A/M/X materials together with amatrix material.

Where the photoactive material of the invention comprises a matrixmaterial, the photoactive material typically comprises a crystallineA/M/X material(s) of the invention suspended in one or more matrixmaterials. The crystalline A/M/X material in this embodiment may be inpowder form, for instance in the form of nanocrystals. The crystallineA/M/X material in this embodiment may be a precipitate obtained directlyfrom the process of the invention, for instance a washed precipitate.Alternatively, it may be obtained by processing a thin film obtained orobtainable by the process of the invention. For example, the thin filmmay be scraped off a substrate and optionally ground to a producecrystals of a smaller size before it is combined with a matrix material.

Suitable matrix materials are described in WO 2017/017441, the entirecontents of which is incorporated by reference herein. A matrix materialis any suitable material in which a plurality of nanoparticles can besuspended. The matrix material is typically solid. The matrix materialis typically non-reactive in that it does not undergo a chemicalreaction with the nanoparticles or any other part of the light emittingdevice (e.g. a metal component). The matrix material typically has ahigh transparency to light across a large proportion of the visiblespectrum.

The matrix material may be an inorganic material or an organic material.The matrix material is usually stable at temperatures up to 150° C. orup to 100° C. Typically, the matrix material comprises a polymericmatrix material.

A polymeric matrix material is a matrix material comprising a polymer.The polymeric matrix material typically comprises a polymer which is apolyalkene (e.g. polyethene, polypropene, polybutene,polymethylmethacrylate or polystyrene), a polyester (e.g. polyethyleneterephthalate, polyhydroxybutyrate or polyethylene apidate), apolyurethane, a polycarbonate, a polyimide, a polyamide (e.g. polyamide6 or polyamide 66), or an epoxy resin. Preferably, the polymeric matrixmaterial comprises a polymer selected from polymethylmethacrylate,polystyrene, polyurethane, a polycarbonate, a polyimide, a polyamide oran epoxy resin.

An inorganic matrix material is typically an inorganic oxide, forexample a metal oxide. Examples of inorganic matrix materials includeZnO, NiO and SnO₂.

In some embodiments, the matrix material is a semiconducting material.Suitable examples of semiconducting matrix materials include poly vinylcarbazole, a polyfluorene derivative, and CBP(4,4′-Bis(N-carbazolyl)-1,1′-biphenyl).

Thus, in some embodiments, the photoactive material comprisesnanoparticles of one or more crystalline A/M/X material(s) of theinvention together with a matrix material selected frompolymethylmethacrylate, polystyrene, polyurethane, a polycarbonate, apolyimide, a polyamide, an epoxy resin, ZnO, NiO and SnO₂, poly vinylcarbazole, a polyfluorene derivative, and CBP(4,4′-Bis(N-carbazolyl)-1,1′-biphenyl).

Where the photoactive material comprises a matrix material, thephotoactive material typically comprises up to 50% by weight of thematrix material. For example, the photoactive material typicallycomprises up to 40% by weight, up to 30% by weight or up to 20% byweight of the matrix material. In such embodiments, the photoactivematerial typically comprises at least 50% by weight of a crystallineA/M/X material(s).

Where the photoactive material comprises a matrix material, thephotoactive material is typically present in the form of a layer. Thethickness of the said layer is typically from 100 nm to 4 mm, forinstance from 1 µm to 1000 µm or from 50 µm to 500 µm. In some cases thelayer may have a thickness of from 1 to 4 mm, for instance if afree-standing layer is to be constructed.

In some embodiments, the photoactive material comprises a scaffoldmaterial. A scaffold material is typically a solid material. A scaffoldmaterial is typically a solid support on which one or more crystallineA/M/X material(s) are distributed.

In some embodiments, the photoactive material of the invention comprisesa porous scaffold together with one or more crystalline A/M/Xmaterial(s). Typically, the said porous scaffold material is in contactwith the said one or more crystalline A/M/X material(s). Suitableexamples of a porous scaffold material are described in WO 2013/171518,the entire contents of which is incorporated by reference.

Typically, a crystalline A/M/X material is disposed on the surface ofthe scaffold material. Typically, where the scaffold is a porousscaffold, a crystalline A/M/X material is disposed on the surface of theporous scaffold material, so that it is supported on the surfaces ofpores within the scaffold. In such embodiments, the crystalline A/M/Xmaterial(s) are distributed on an internal surface or surfaces of thescaffold material so that the said material(s) may be said to bedistributed within the scaffold material. The crystalline A/M/Xmaterial(s) in this situation typically act as a light-absorbing,photosensitising materials, as well as charge-transporting materials. Aphotoactive material comprising a crystalline A/M/X material and ascaffold material, e.g. a porous scaffold material, can advantageouslyrapidly transport charge carriers through the photoactive material. Thecrystalline A/M/X material(s) may typically be deposited on the scaffoldmaterial by a process for producing a thin film as described hereinwherein the scaffold material is a thin layer present upon thesubstrate.

A scaffold material is typically a dielectric scaffold material or ann-type scaffold material. Preferably, a scaffold material is a porousdielectric scaffold material or a porous n-type scaffold material.

By “dielectric material” is meant an insulating material. A dielectricscaffold material may comprise a material having a band gap greater thanor equal to 3.6 eV, or greater than or equal to 4 eV. A dielectricscaffold material is often a dielectric oxide. A dielectric scaffoldmaterial typically comprises one or more oxides of aluminium, germanium,zirconium, silicon, yttrium or ytterbium. However, a dielectric scaffoldmaterial may comprise one or more of polymethylmethacrylate (PMMA),polystyrene, polycarbonate, or polyimide. A dielectric scaffold materialmay be preferably be selected from aluminasilicate, zirconia, aluminaand silica, for instance alumina (Al₂O₃).

An n-type scaffold may be selected from any n-type material describedherein, for instance titania (TiO₂). By “n-type material” is meant anelectron-transporting material.

A suitable n-type material is typically an inorganic material. Asuitable n-type inorganic material may be selected from a metal oxide, ametal sulphide, a metal selenide, a metal telluride, a perovskite,amorphous Si, an n-type group IV semiconductor, an n-type group III-Vsemiconductor, an n-type group II-VI semiconductor, an n-type groupI-VII semiconductor, an n-type group IV-VI semiconductor, an n-typegroup V-VI semiconductor, and an n-type group II-V semiconductor, any ofwhich may be doped or undoped. More typically, the n-type material isselected from a metal oxide, a metal sulphide, a metal selenide, and ametal telluride. The n-type material may comprise an inorganic materialselected from oxide of titanium, tin, zinc, niobium, tantalum, tungsten,indium, gallium, neodinium, palladium, or cadmium, or an oxide of amixture of two or more of said metals. For instance, the n-type materialmay comprise TiO₂, SnO₂, ZnO, Nb₂O₅, Ta₂O₅, WO₃, W₂O₅, In₂O₃, Ga₂O₃,Nd₂O₃, PbO, or CdO. Other suitable n-type materials that may be employedinclude sulphides of cadmium, tin, copper, or zinc, including sulphidesof a mixture of two or more of said metals. For instance, the sulphidemay be FeS₂, CdS, ZnS or Cu₂ZnSnS₄. The n-type material may for instancecomprise a selenide of cadmium, zinc, indium, or gallium or a selenideof a mixture of two or more of said metals; or a telluride of cadmium,zinc, cadmium or tin, or a telluride of a mixture of two or more of saidmetals. For instance, the selenide may be Cu(In,Ga)Se₂. Typically, thetelluride is a telluride of cadmium, zinc, cadmium or tin. For instance,the telluride may be CdTe. The n-type material may for instance comprisean inorganic material selected from oxide of titanium, tin, zinc,niobium, tantalum, tungsten, indium, gallium, neodinium, palladium,cadmium, or an oxide of a mixture of two or more of said metals; asulphide of cadmium, tin, copper, zinc or a sulphide of a mixture of twoor more of said metals; a selenide of cadmium, zinc, indium, gallium ora selenide of a mixture of two or more of said metals; or a telluride ofcadmium, zinc, cadmium or tin, or a telluride of a mixture of two ormore of said metals. Examples of other semiconductors that may besuitable n-type materials, for instance if they are n-doped, includegroup IV compound semiconductors; amorphous Si; group III-Vsemiconductors (e.g. gallium arsenide); group II-VI semiconductors (e.g.cadmium selenide); group I-VII semiconductors (e.g. cuprous chloride);group IV-VI semiconductors (e.g. lead selenide); group V-VIsemiconductors (e.g. bismuth telluride); and group II-V semiconductors(e.g. cadmium arsenide). Typically, the n-type material comprises TiO₂.

Thus, in some embodiments the photoactive material of the invention maycomprise a crystalline A/M/X material together with a porous scaffoldmaterial, preferably wherein the dielectric scaffold material isselected from one or more of alumina, silica and titania.

Where the photoactive material comprises a scaffold material, thephotoactive material is typically present in the form of a layer.Typically, the thickness of the said layer is from 200 nm to 1000 nm,for instance the thickness may be from 400 nm to 800 nm. Often,thickness of the said layer is from 400 nm to 600 nm.

A layer of the photoactive material of the invention may be without openporosity. The layer of the photoactive material may comprise a layer ofa crystalline A/M/X material, which layer of a crystalline A/M/Xmaterial is without open porosity. Alternatively, the layer of thephotoactive material may be porous, or the layer of the photoactivematerial may be deposited on a porous scaffold as described herein.

Optoelectronic Device

The invention further provides an optoelectronic device comprising aphotoactive material as defined herein. Thus, the invention provides anoptoelectronic device comprising a crystalline A/M/X material obtainableor obtained by a process as defined herein. In a particularly preferredembodiment, the invention provides an optoelectronic device comprising athin film of a crystalline A/M/X material obtainable or obtained by aprocess as defined herein.

The term “optoelectronic device” as used herein, refers to devices whichsource, control, detect or emit light. Typically, the light is visiblelight, e.g. light having a wavelength of about 450 to 700 nm. In apreferred embodiment, the device is capable of emitting blue light (i.e.light having a wavelength of about 450 to 500 nm, preferably about 455to 495 nm, more preferably about 460 to 490 nm). The optoelectronicdevice of the invention comprises the photoactive material of theinvention and is therefore capable of one or more of:

-   (i) absorbing photons, which may then generate free charge carriers    e.g. electrons and holes;-   (ii) absorbing photons at energies higher than its band gap and    re-emit photons at energies of the band gap; and-   (iii) accepting charge, both electrons and holes, which may    subsequently recombine and emit light.

In one preferred embodiment, the invention provides an optelectronicdevice which is a photovoltaic device, comprising a photoactive materialas defined herein. By “photovoltaic device” is meant herein a devicethat is capable of converting electrical energy into light, particularlyvisible light. In a preferred embodiment the said photovoltaic device iscapable of converting electrical energy into blue light (i.e. lighthaving a wavelength of about 450 to 500 nm, preferably about 455 to 495nm, more preferably about 460 to 490 nm).

In another preferred embodiment, the invention provides an optelectronicdevice which is a light-emitting device, comprising a photoactivematerial as defined herein. The light-emitting device may be alight-emitting diode (LED). The light-emitting device of the inventionis typically capable of emitting visible light. In a preferredembodiment the light-emitting device is capable of emitting blue light(i.e. light having a wavelength of about 450 to 500 nm, preferably about455 to 495 nm, more preferably about 460 to 490 nm).

In some preferred embodiments, the optoelectronic device of theinvention comprises a photoactive material, wherein the photoactivematerial is disposed in a layer. A layer of photoactive material isdefined herein. Usually, the said layer of photoactive material has athickness of at least 2 nm. However, in some embodiments where the layeris intended to stand alone, the layer may be considerably thicker (forexample up to 10 mm thick, more usually up to 5 mm thick).

In some embodiments, the optoelectronic device of the inventioncomprises a layer of photoactive material which is a thin sensitisinglayer, for instance having a thickness of from 5 nm to 50 nm.

In devices wherein the layer of said photoactive material forms a planarheterojunction with an n-type or p-type region, the layer of saidphotoactive material may have a thickness of at least 1 nm, for instanceat least 10 nm or at least 50 nm, or for example a thickness of greaterthan or equal to 100 nm. Preferably, the layer of said photoactivematerial has a thickness of from 100 nm to 700 nm, for instance from 200nm to 500 nm. The term “planar heterojunction”, as used herein, meansthat surface defining junction between the semiconducting material andthe n- or p-type region is substantially planar and has a low roughness,for instance a root mean squared roughness of less than 20 nm over anarea of 25 nm by 25 nm, for instance a root mean squared roughness ofless than 10 nm, or less than 5 nm, over an area of 25 nm by 25 nm.

The photoactive material often acts as a photoactive component (e.g. aphotoabsorbent component or a photoemissive component) within theoptoelectronic device. In other embodiments, the semiconducting materialmay form a layer of a p-type or n-type semiconductor in anoptoelectronic device, e.g. a solar cell or an LED.

Typically, the optoelectronic device of the invention (which may be alight emitting device or a photovoltaic device) comprises:

-   (a) an n-type region comprising at least one n-type layer;-   (b) a p-type region comprising at least one p-type layer; and,    disposed between the n-type region and the p-type region:-   (c) the layer of the photoactive material.

Preferred examples of the optoelectronic device of the invention (whichmay be a light emitting device or a photovoltaic device) include an LED,a photodiode, a solar cell, a photodetector, or a photosensor;particularly preferable are an LED, a photodiode or a solar cell.

For instance, the optoelectronic device may comprise:

-   an n-type region comprising at least one n-type layer;-   a p-type region comprising at least one p-type layer; and, disposed    between the n-type region and the p-type region:-   said layer of a photoactive material which comprises (or consists    essentially of) a crystalline A/M/X material as defined herein.

The n-type region comprises at least one n-type layer. The n-type regiontypically comprises one or two n-type layers. Each layer may be porousor compact. A compact layer is typically a layer without open porosity(e.g. absent of any meso- or macroporosity). The p-type region comprisesat least one p-type layer. The p-type region typically comprises one ortwo p-type layers. Each layer may be porous or compact. A compact layeris typically a layer without open porosity. It may be a non-porouslayer. The n-type and p-type materials, in these layers, may be asfurther defined herein.

In some cases, the optoelectronic device comprises a layer of saidphotoactive material without open porosity. The layer of saidphotoactive material without open porosity is typically a crystallinelayer of a crystalline A/M/X material (typically a thin film, preferablya perovskite) according to the invention without open porosity. Thus,the layer of said photoactive material may comprise greater than orequal to 95 volume% of one or more crystalline A/M/X material(s) (andthus less than 5 volume% of absence pore volume). The layer without openporosity typically does not comprise macropores or mesopores.

The layer of the photoactive material typically forms a planarheterojunction with the n-type region or the p-type region. The layer ofthe photoactive material typically forms a first planar heterojunctionwith the n-type region and a second planar heterojunction with thep-type region. This forms a planar heterojunction device, of the kinddescribed in WO 2014/045021. The term “planar heterojunction” as usedherein refers to a junction between two regions where one region doesnot infiltrate the other. This does not require that the junction iscompletely smooth, just that one region does not substantiallyinfiltrate pores in the other region.

When the layer of the photoactive material forms a planar heterojunctionwith both the p-type and the n-type region, this typically forms a thinfilm device. The thickness of the layer of the photoactive material maybe greater than or equal to 50 nm.

In some embodiments, it is desirable to have a porous scaffold materialpresent, wherein the porous scaffold is as defined herein. The scaffoldmaterial may aid charge transport from the photoactive material to anadjacent region. The scaffold material may also, or alternatively, aidformation of the layer of the photoactive material during deviceconstruction. Thus, in some embodiments, the optoelectronic devicecomprises:

-   an n-type region comprising at least one n-type layer;-   a p-type region comprising at least one p-type layer; and, disposed    between the n-type region and the p-type region, a layer of    photoactive material comprising:    -   (i) a porous scaffold material; and    -   (ii) a crystalline A/M/X material in contact with the scaffold        material.

The architecture of such devices is described in more detail in WO2014/045021, whose entire contents are incorporated herein by reference.

The layer of photoactive material comprising a porous scaffold materialand a crystalline A/M/X material may form a sensitizing layer of thephotoactive material. Thus, the optoelectronic device may be asensitized device.

Further details of the architecture of optoelectronic devices such assolar cells and suitable materials therefor are described in publishedapplication WO 2017/037448, the entire contents of which is incorporatedherein by reference. The crystalline A/M/X material of the presentinvention may be used in place of the semiconducting material therein.

In some embodiments, the optoelectronic device comprises:

-   an n-type region comprising at least one n-type layer;-   a p-type region comprising at least one p-type layer; and, disposed    between the n-type region and the p-type region:    -   (i) a first layer which comprises a porous scaffold material and        said crystalline A/M/X material; and    -   (ii) a capping layer disposed on said first layer, which capping        layer is a layer of said crystalline A/M/X material without open        porosity,-   wherein crystalline A/M/X material in the capping layer is in    contact with the crystalline A/M/X material in the first layer.

The first layer comprises said porous scaffold material and saidcrystalline A/M/X material disposed on the surface of the scaffoldmaterial. The term “scaffold material” as used herein refers to amaterial whose function(s) include acting as a physical support foranother material. In the present case, the scaffold material acts as asupport for the crystalline A/M/X material present in the first layer.The crystalline A/M/X material is disposed, or supported on, the surfaceof the scaffold material. The porous scaffold material typically has anopen porous structure. Accordingly, the “surface” of the porous scaffoldmaterial here typically refers to the surfaces of pores within thescaffold material. Thus, the crystalline A/M/X material in the firstlayer is typically disposed on the surfaces of pores within the scaffoldmaterial.

In some embodiments, the scaffold material is porous and the crystallineA/M/X material in the first layer is disposed in pores of the scaffoldmaterial. The effective porosity of said scaffold material is usually atleast 50%. For instance, the effective porosity may be about 70%. In oneembodiment, the effective porosity is at least 60%, for instance atleast 70%.

Typically, the crystalline A/M/X material (or photoactive material) inthe first layer contacts one of the p-type and n-type regions, and thecrystalline A/M/X material in the capping layer contacts the other ofthe p-type and n-type regions. The crystalline A/M/X material in thecapping layer typically forms a planar heterojunction with the p-typeregion or the n-type region.

In one embodiment, the crystalline A/M/X material in the capping layercontacts the p-type region, and the crystalline A/M/X material in thefirst layer contacts the n-type region. In another embodiment, thecrystalline A/M/X material in the capping layer contacts the n-typeregion, and the crystalline A/M/X material in the first layer contactsthe p-type region (for instance in an inverted device).

In one embodiment, the crystalline A/M/X material in the capping layercontacts the p-type region, and the crystalline A/M/X material in thefirst layer contacts the n-type region. Usually, in this embodiment, thescaffold material is either an electron-transporting scaffold materialor a dielectric scaffold material. Typically, the crystalline A/M/Xmaterial in the capping layer forms a planar heterojunction with thep-type region.

In another embodiment, however, the crystalline A/M/X material in thecapping layer contacts the n-type region, and the crystalline A/M/Xmaterial in the first layer contacts the p-type region. Typically, inthis embodiment, the scaffold material is a hole-transporting scaffoldmaterial or a dielectric scaffold material. Typically, the crystallineA/M/X material in the capping layer forms a planar heterojunction withthe n-type region.

The thickness of the capping layer is usually greater than the thicknessof the first layer. The majority of any photoactivity (e.g. lightabsorption or light emission) therefore usually occurs in a cappinglayer. The thickness of the capping layer is typically from 10 nm to 100µm. More typically, the thickness of the capping layer is from 10 nm to10 µm. Preferably, the thickness of the capping layer is from 50 nm to1000 nm, or for instance from 100 nm to 700 nm. The thickness of thecapping layer may be greater than or equal to 100 nm. The thickness ofthe first layer, on the other hand, is often from 5 nm to 1000 nm. Moretypically, it is from 5 nm to 500 nm, or for instance from 30 nm to 200nm.

The n-type region is typically an n-type layer. The n-type region mayalternatively comprise an n-type layer and an n-type exciton blockinglayer. Such an n-type exciton blocking layer is typically disposedbetween the n-type layer and the layer(s) comprising the crystallineA/M/X material. The n-type region may have a thickness of from 50 nm to1000 nm. For instance, the n-type region may have a thickness of from 50nm to 500 nm, or from 100 nm to 500 nm.

Preferably, the n-type region comprises a compact layer of an n-typesemiconductor. The n-type region may further comprise a porous layer ofan n-type semiconductor which may be the porous scaffold material asdescribed above (wherein the porous scaffold material is anelectron-transporting material).

The n-type region in the optoelectronic device of the inventioncomprises one or more n-type layers. Often, the n-type region is ann-type layer, i.e. a single n-type layer. In other embodiments, however,the n-type region may comprise an n-type layer and an n-type excitonblocking layer. In cases where an n-type exciton blocking layer isemployed, the n-type exciton blocking layer is usually disposed betweenthe n-type layer and the layer(s) comprising the crystalline A/M/Xmaterial.

An exciton blocking layer is a material which is of wider band gap thanthe crystalline A/M/X material, but has either its conduction band orvalance band closely matched with those of the crystalline A/M/Xmaterial. If the conduction band (or lowest unoccupied molecular orbitalenergy levels) of the exciton blocking layer are closely aligned withthe conduction band of the crystalline A/M/X, then electrons can passfrom the crystalline A/M/X material into and through the excitonblocking layer, or through the exciton blocking layer and into thecrystalline A/M/X material, and we term this an n-type exciton blockinglayer. An example of such is bathocuproine, as described in P. Pcumans,A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-filmphotodetectors and solar cells” J. Appl. Phys. 93, 3693 (2001) andMasaya Hirade, and Chihaya Adachi, “Small molecular organic photovoltaiccells with exciton blocking layer at anode interface for improved deviceperformance” Appl. Phys. Lett. 99, 153302 (2011)}.

An n-type layer is a layer of an electron-transporting (i.e. an n-type)material. The n-type material may, for instance, be a single n-typecompound or elemental material, which may be undoped or doped with oneor more dopant elements.

The n-type layer employed in the optoelectronic device of the inventionmay comprise an inorganic or an organic n-type material.

A suitable inorganic n-type material may be selected from a metal oxide,a metal sulphide, a metal selenide, a metal telluride, a perovskite,amorphous Si, an n-type group IV semiconductor, an n-type group III-Vsemiconductor, an n-type group II-VI semiconductor, an n-type groupI-VII semiconductor, an n-type group IV-VI semiconductor, an n-typegroup V-VI semiconductor, and an n-type group II-V semiconductor, any ofwhich may be doped or undoped.

The n-type material may be selected from a metal oxide, a metalsulphide, a metal selenide, a metal telluride, amorphous Si, an n-typegroup IV semiconductor, an n-type group III-V semiconductor, an n-typegroup II-VI semiconductor, an n-type group I-VII semiconductor, ann-type group IV-VI semiconductor, an n-type group V-VI semiconductor,and an n-type group 11-V semiconductor, any of which may be doped orundoped.

More typically, the n-type material is selected from a metal oxide, ametal sulphide, a metal selenide, and a metal telluride.

Thus, the n-type layer may comprise an inorganic material selected fromoxide of titanium, tin, zinc, niobium, tantalum, tungsten, indium,gallium, neodymium, palladium, or cadmium, or an oxide of a mixture oftwo or more of said metals. For instance, the n-type layer may compriseTiO₂, SnO₂, ZnO, Nb₂O₅, Ta₂O₅, WO₃, W₂O₅, In₂O₃, Ga₂O₃, Nd₂O₃, PbO, orCdO. Other suitable n-type materials that may be employed includesulphides of cadmium, tin, copper, or zinc, including sulphides of amixture of two or more of said metals. For instance, the sulphide may beFeS₂, CdS, ZnS, SnS, BiS, SbS, or Cu₂ZnSnS₄.

The n-type layer may for instance comprise a selenide of cadmium, zinc,indium, or gallium or a selenide of a mixture of two or more of saidmetals; or a telluride of cadmium, zinc, cadmium or tin, or a tellurideof a mixture of two or more of said metals. For instance, the selenidemay be Cu(In,Ga)Se₂. Typically, the telluride is a telluride of cadmium,zinc, cadmium or tin. For instance, the telluride may be CdTe.

The n-type layer may for instance comprise an inorganic materialselected from oxide of titanium, tin, zinc, niobium, tantalum, tungsten,indium, gallium, neodymium, palladium, cadmium, or an oxide of a mixtureof two or more of said metals; a sulphide of cadmium, tin, copper, zincor a sulphide of a mixture of two or more of said metals; a selenide ofcadmium, zinc, indium, gallium or a selenide of a mixture of two or moreof said metals; or a telluride of cadmium, zinc, cadmium or tin, or atelluride of a mixture of two or more of said metals. Examples of othersemiconductors that may be suitable n-type materials, for instance ifthey are n-doped, include group IV elemental or compound semiconductors;amorphous Si; group III-V semiconductors (e.g. gallium arsenide); groupII-VI semiconductors (e.g. cadmium selenide); group I-VII semiconductors(e.g. cuprous chloride); group IV-VI semiconductors (e.g. leadselenide); group V-VI semiconductors (e.g. bismuth telluride); and groupII-V semiconductors (e.g. cadmium arsenide).

Typically, the n-type layer comprises TiO₂.

When the n-type layer is an inorganic material, for instance TiO₂ or anyof the other materials listed above, it may be a compact layer of saidinorganic material. Preferably the n-type layer is a compact layer ofTiO₂.

Other n-type materials may also be employed, including organic andpolymeric electron-transporting materials, and electrolytes. Suitableexamples include, but are not limited to a fullerene or a fullerenederivative (for instance C₆₀ or Phenyl-C61-butyric acid methyl ester(PCBM)), an organic electron transporting material comprising peryleneor a derivative thereof, orpoly{[N,NO-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20-bithiophene)}(P(NDI2OD-T2)).

The p-type region is typically a p-type layer. The p-type region mayalternatively comprise an p-type layer and a p-type exciton blockinglayer. Such a p-type exciton blocking layer is typically disposedbetween the p-type layer and the layer(s) comprising the crystallineA/M/X material. The p-type region may have a thickness of from 50 nm to1000 nm. For instance, the p-type region may have a thickness of from 50nm to 500 nm, or from 100 nm to 500 nm.

The p-type region in the optoelectronic device of the inventioncomprises one or more p-type layers. Often, the p-type region is ap-type layer, i.e. a single p-type layer. In other embodiments, however,the p-type region may comprise a p-type layer and a p-type excitonblocking layer. In cases where a p-type exciton blocking layer isemployed, the p-type exciton blocking layer is usually disposed betweenthe p-type layer and the layer(s) comprising the crystalline A/M/Xmaterial. If the valence band (or highest occupied molecular orbitalenergy levels) of the exciton blocking layer is closely aligned with thevalence band of the crystalline A/M/X material, then holes can pass fromthe crystalline A/M/X material into and through the exciton blockinglayer, or through the exciton blocking layer and into the crystallineA/M/X material, and we term this a p-type exciton blocking layer. Anexample of such is tris[4-(5-phenylthiophen-2-yl)phenyl]amine, asdescribed in Masaya Hirade, and Chihaya Adachi, “Small molecular organicphotovoltaic cells with exciton blocking layer at anode interface forimproved device performance” Appl. Phys. Lett. 99, 153302 (2011).

A p-type layer is a layer of a hole-transporting (i.e. a p-type)material. The p-type material may be a single p-type compound orelemental material, or a mixture of two or more p-type compounds orelemental materials, which may be undoped or doped with one or moredopant elements.

The p-type layer employed in the optoelectronic device of the inventionmay comprise an inorganic or an organic p-type material. Typically, thep-type region comprises a layer of an organic p-type material.

Suitable p-type materials may be selected from polymeric or molecularhole transporters. The p-type layer employed in the optoelectronicdevice of the invention may for instance comprise spiro-OMeTAD(2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorenc)),P3HT (poly(3-hexylthiophone)), PCPDTBT(Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]]),PVK (poly(N-vinylcarbazole)), HTM-TFSI (1-hexyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide), Li-TFSI (lithiumbis(trifluoromethanesulfonyl)imide) or tBP (tert-butylpyridine). Thep-type region may comprise carbon nanotubes. Usually, the p-typematerial is selected from spiro-OMeTAD, P3HT, PCPDTBT and PVK.Preferably, the p-type layer employed in the optoelectronic device ofthe invention comprises spiro-OMeTAD.

The p-type layer may for example comprise spiro-OMeTAD(2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene)),P3HT (poly(3-hexylthiophene)), PCPDTBT(Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]]),or PVK (poly(N-vinylcarbazole)).

Suitable p-type materials also include molecular hole transporters,polymeric hole transporters and copolymer hole transporters. The p-typematerial may for instance be a molecular hole transporting material, apolymer or copolymer comprising one or more of the following moieties:thiophenyl, phenelenyl, dithiazolyl, benzothiazolyl,diketopyrrolopyrrolyl, ethoxydithiophenyl, amino, triphenyl amino,carbozolyl, ethylene dioxythiophenyl, dioxythiophenyl, or fluorenyl.Thus, the p-type layer employed in the optoelectronic device of theinvention may for instance comprise any of the aforementioned molecularhole transporting materials, polymers or copolymers.

Suitable p-type materials also include m-MTDATA(4,4′,4″-tris(methylphenylphenylamino)triphenylamine), MeOTPD(N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine), BP2T(5,5′-di(biphenyl-4-yl)-2,2′-bithiophene), Di-NPB(N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine),α-NPB (N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine), TNATA(4,4′,4″-tris-(N-(naphthylen-2-yl)-N-phenylamine)triphenylamine), BPAPF(9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene), spiro-NPB(N2,N7-Di-1-naphthalenyl-N2,N7-diphenyl-9,9′-spirobi[9H-fluorene]-2,7-diamine),4P-TPD (4,4-bis-(N,N-diphenylamino)-tetraphenyl), PEDOT:PSS andspiro-OMeTAD.

The p-type layer may be doped, for instance with tertbutyl pyridine andLiTFSI. The p-type layer may be doped to increase the hole-density. Thep-type layer may for instance be doped with NOBF₄ (Nitrosoniumtetrafluoroborate), to increase the hole-density.

In other embodiments, the p-type layer may comprise an inorganic holetransporter. For instance, the p-type layer may comprise an inorganichole transporter comprising an oxide of nickel, vanadium, copper ormolybdenum; CuI, CuBr, CuSCN, Cu₂O, CuO or CIS; a perovskite; amorphousSi; a p-type group IV semiconductor, a p-type group III-V semiconductor,a p-type group II-VI semiconductor, a p-type group I-VII semiconductor,a p-type group IV-VI semiconductor, a p-type group V-VI semiconductor,and a p-type group II-V semiconductor, which inorganic material may bedoped or undoped. The p-type layer may be a compact layer of saidinorganic hole transporter.

The p-type layer may for instance comprise an inorganic hole transportercomprising an oxide of nickel, vanadium, copper or molybdenum; CuI,CuBr, CuSCN, Cu₂O, CuO or CIS; amorphous Si; a p-type group IVsemiconductor, a p-type group III-V semiconductor, a p-type group II-VIsemiconductor, a p-type group I-VII semiconductor, a p-type group IV-VIsemiconductor, a p-type group V-VI semiconductor, and a p-type groupII-V semiconductor, which inorganic material may be doped or undoped.The p-type layer may for instance comprise an inorganic hole transporterselected from CuI, CuBr, CuSCN, Cu₂O, CuO and CIS. The p-type layer maybe a compact layer of said inorganic hole transporter.

Typically, the p-type layer comprises a polymeric or molecular holetransporter, and the n-type layer comprises an inorganic n-typematerial. The p-type polymeric or molecular hole transporter may be anysuitable polymeric or molecular hole transporter, for instance any ofthose listed above. Likewise, the inorganic n-type material may be anysuitable n-type inorganic, for instance any of those listed above. Inone embodiment, for instance, the p-type layer comprises spiro-OMeTADand the n-type layer comprises TiO₂. Typically, in that embodiment, then-type layer which comprises TiO₂ is a compact layer of TiO₂.

In other embodiments, both the n-type layer and the p-type layercomprise inorganic materials. Thus, the n-type layer may comprise aninorganic n-type material and the p-type layer may comprise an inorganicp-type material. The inorganic p-type material may be any suitablep-type inorganic, for instance any of those listed above. Likewise, theinorganic n-type material may be any suitable n-type inorganic, forinstance any of those listed above. In other embodiments, the p-typelayer comprises an inorganic p-type material (i.e. an inorganic holetransporter) and the n-type layer comprises a polymeric or molecularhole transporter. The inorganic p-type material may be any suitablep-type inorganic, for instance any of those listed above. Likewise, then-type polymeric or molecular hole transporter may be any suitablen-type polymeric or molecular hole transporter, for instance any ofthose listed above.

For instance, the p-type layer may comprise an inorganic holetransporter and the n-type layer may comprise an electron transportingmaterial, wherein the electron transporting material comprises afullerene or a fullerene derivative, an electrolyte, or an organicelectron transporting material, preferably wherein the organic electrontransporting material comprises perylene or a derivative thereof, orpoly {[N,N0-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20-bithiophene)}(P(NDI2OD-T2)). The inorganic hole transporter may for instance comprisean oxide of nickel, vanadium, copper or molybdenum; CuI, CuBr, CuSCN,Cu₂O, CuO or CIS; a perovskite; amorphous Si; a p-type group IVsemiconductor, a p-type group III-V semiconductor, a p-type group II-VIsemiconductor, a p-type group I-VII semiconductor, a p-type group IV-VIsemiconductor, a p-type group V-VI semiconductor, and a p-type groupII-V semiconductor, which inorganic material may be doped or undoped.More typically, the inorganic hole transporter comprises an oxide ofnickel, vanadium, copper or molybdenum; CuI, CuBr, CuSCN, Cu₂O, CuO orCIS; a p-type group IV semiconductor, a p-type group III-Vsemiconductor, a p-type group II-VI semiconductor, a p-type group I-VIIsemiconductor, a p-type group IV-VI semiconductor, a p-type group V-VIsemiconductor, and a p-type group 11-V semiconductor, which inorganicmaterial may be doped or undoped. Thus, the inorganic hole transportermay comprise an oxide of nickel, vanadium, copper or molybdenum; CuI,CuBr, CuSCN, Cu₂O, CuO or CIS.

The optoelectronic device device typically further comprises one or morefirst electrodes and one or more second electrodes. The one or morefirst electrodes are typically in contact with the n-type region, ifsuch a region is present. The one or more second electrodes aretypically in contact with the p-type region, if such a region ispresent. Typically: the one or more first electrodes are in contact withthe n-type region and the one or more second electrodes are in contactwith the p-type region; or the one or more first electrodes are incontact with the p-type region and the one or more second electrodes arein contact with the n-type region. The first and second electrode maycomprise any suitable electrically conductive material. The firstelectrode typically comprises a transparent conducting oxide. The secondelectrode typically comprises one or more metals. Typically, the firstelectrode comprises a transparent conducting oxide and the secondelectrode typically comprises one or more metals.

The transparent conducting oxide may be as defined above and is oftenFTO, ITO, or AZO, and typically ITO. The metal may be any metal.Generally the second electrode comprises a metal selected from silver,gold, copper, aluminium, platinum, palladium, or tungsten. Theelectrodes may form a single layer or may be patterned.

An optoelectronic device according to the invention, for instance alight emitting device or a photovoltaic device, may comprise thefollowing layers in the following order:

-   I. one or more first electrodes as defined herein;-   II. optionally a compact n-type layer as defined herein;-   III. a porous layer of an n-type material as defined herein;-   IV. a layer of said crystalline A/M/X material (e.g.as a    sensitizer), preferably a thin film of said crystalline A/M/X    material;-   V. a p-type region as defined herein;-   VI. optionally a further compact p-type layer as defined herein; and-   VII. one or more second electrodes as defined herein.

An optoelectronic device according to the invention, for instance alight emitting device or a photovoltaic device may comprise thefollowing layers in the following order:

-   I. one or more first electrodes as defined herein;-   II. an n-type region comprising at least one n-type layer as defined    herein;-   III. a layer of the material comprising the crystalline A/M/X    material (preferably a thin film of the crystalline A/M/X material)    as defined herein;-   IV. a p-type region comprising at least one p-type layer as defined    herein; and-   V. one or more second electrodes as defined herein.

An optoelectronic device according to the invention may comprise thefollowing layers in the following order:

-   I. one or more first electrodes which comprise a transparent    conducting oxide, preferably FTO;-   II. an n-type region comprising at least one n-type layer as defined    herein;-   III. a layer of the crystalline A/M/X material (preferably a layer    comprising a thin film of the crystalline A/M/X material) as defined    herein;-   IV. a p-type region comprising at least one p-type layer as defined    herein; and-   V. one or more second electrodes which comprise a metal, preferably    silver or gold.

An optoelectronic device (for instance an inverted device) according tothe invention may comprise the following layers in the following order:

-   I. one or more second electrodes as defined herein;-   II. an n-type region comprising at least one n-type layer as defined    herein;-   III. a layer of the crystalline A/M/X material (preferably a layer    comprising a thin film of the crystalline A/M/X material) as defined    herein;-   IV. a p-type region comprising at least one p-type layer as defined    herein; and-   V. one or more first electrodes as defined herein.

An optoelectronic device according to the invention, for instance alight emitting device or a photovoltaic device, may comprise thefollowing layers in the following order

-   I. one or more second electrodes which comprises a metal;-   II. an n-type region comprising at least one mesoporous n-type layer    as defined herein;-   III. a sensitising layer of the crystalline A/M/X material    (preferably a sensitising layer comprising a thin film of the    crystalline A/M/X material) as defined herein;-   IV. a p-type region comprising at least one p-type layer as defined    herein; and-   V. one or more first electrodes which comprise a transparent    conducting oxide.

The one or more first electrodes may have a thickness of from 100 nm to700 nm, for instance of from 100 nm to 400 nm. The one or more secondelectrodes may have a thickness of from 10 nm to 500 nm, for instancefrom 50 nm to 200 nm or from 10 nm to 50 nm. The n-type region may havea thickness of from 50 nm to 500 nm. The p-type region may have athickness of from 50 nm to 500 nm.

Many of the above-described optoelectronic devices comprise a layer of acrystalline A/M/X material (preferably a layer comprising a thin film ofthe crystalline A/M/X material) disposed on a preceding layer in thedevice (for example a layer of semiconducting material, a layer of ann-type semiconductor, a layer of a p-type semiconductor, a scaffoldmaterial etc.). In these cases, the layer of crystalline A/M/X materialin the device may be prepared as described herein. The optoelectronicdevices may also further comprise subsequent layers disposed on thelayer of crystalline A/M/X material. Subsequent layers disposed on thelayer of crystalline A/M/X material may be prepared as described in WO2017/037448.

In other embodiments, the photoactive material functions as a phosphorin the optoelectronic device. In such embodiments, the optoelectronicdevice typically comprises a light source and a photoactive material asdefined herein. Typically, the photoactive material comprises ananocrystals of a crystalline A/M/X material as defined herein,obtainable directly from the precipitate formed by the process of theinvention or obtainable from a thin film produced by a process asdefined herein. Also typically, the photoactive material comprises amatrix material as defined herein.

By “functions as a phosphor” is meant that the photoactive materialfunctions by absorbing a first wavelength of light and subsequentlyre-emitting a different, larger wavelength of light.

The light source is typically a white, blue or UV light source. That is,the light source is typically a source of electromagnetic radiationwhich emits at wavelengths of 500 nm or less, more usually 480 nm orless. For example, the light source typically emits electromagneticradiation with a wavelength of from 400 to 480 nm.

Examples of optoelectronic devices wherein the photoactive materialfunctions as a phosphor include display screens, such as LED displayscreens, and solid-state lighting devices. Such devices represent afurther aspect of the invention.

Use

In a further aspect, the invention provides uses of the crystallineA/M/X materials of the invention and the photoactive materials of theinvention comprising said compounds.

In a first embodiment, the invention provides the use of a crystallineA/M/X material or photoactive material as defined herein as aphoto-emitter, preferably a photo-emitter which emits light, preferablyvisible light (i.e. light having a wavelength in the region 450 to 700nm). In a preferred embodiment, the photo-emitter emits blue light, i.e.light in the wavelength range 450 to 500 nm, more preferably 455 to 495nm, further preferably 460 to 490 nm.

In a second embodiment, the invention provides the use of a crystallineA/M/X material or a photoactive material as defined herein in themanufacture of an optoelectronic device, preferably wherein saidoptoelectronic device emits light, preferably visible light and morepreferably wherein said optoelectronic device emits blue light in thewavelength range 450 to 500 nm, more preferably 455 to 495 nm, furtherpreferably 460 to 490 nm.

In a third embodiment, the invention provides the use of a crystallineA/M/X material or a photoactive material as defined herein as aphosphor. For example, the invention provides the use of a photoactivematerial as defined herein in the manufacture of a screen, in particularan LED screen, or a solid-state lighting device.

In a fourth embodiment, the invention further provides the use of acrystalline A/M/X material or an optoelectronic device as defined hereinin a method of generating light, preferably visible light, morepreferably in a method of generating light in the wavelength range 450to 500 nm, more preferably 455 to 495 nm, further preferably 460 to 490nm. For example, the invention provides a method of generating lightcomprising irradiating a crystalline A/M/X material or a photoactivematerial as defined herein with a light source; and thus generatingemission of light from said photoactive material in the wavelength range450 to 500 nm, more preferably 455 to 495 nm, further preferably 460 to490 nm.

EXAMPLES

The advantages of the invention will hereafter be described withreference to some specific examples.

1. Materials Used in the Examples

CsBr (99.9%), CsCl (99.9%), RbBr (99.9%), RbCl (99.9%), Formamidineacetate salt (FAAc: 99%), PbBr₂ (≥98%), PbCl₂ (≥98%), Pb acetate(Pb(Ac)₂: ≥99.99%), Hydrobromic acid (HBr: 48 wt% in water, or 47% inwater where indicated), N,N-dimethylformamide (DMF: anhydrous 99.8%),dimethyl sulfoxide (DMSO: anhydrous 99.9%), ethanol (laboratory reagent,96%), methanol (anhydrous, 99.8%), isopropanol (anhydrous 99.5%) toluene(anhydrous 99.8%), methyl acetate (anhydrous 99.5%), acetic acid (≥99%),Poly(methyl methacrylate) (PMMA: average Mw 97,000 (Typical), average Mn46,000 (Typical)) were purchased from Sigma-Aldrich. Cs₂CO₃ (99%) andHydriodic acid (HI: 57% w/w aq. soln., stab with 1.5% hypophosphorousacid) were purchased from Alfa Aesar. Hydrochloric Acid (HCl: 32 % inH₂O) was purchased from Fisher Scientific.

2. Fabrication of Cs_(1-x)Rb_(x)PbBr₃ Crystalline Powders

Cs_(1x)Rb_(x)PbBr₃ was prepared by mixing CsBr and RbBr (in variablemolar ratio) combined with PbBr₂ in stoichiometric amounts. For example,Cs_(0.5)Rb_(0.5)PbBr₃ was prepared with 1.064 g (5 mmol) of CsBr and0.827 g (5 mmol) of RbBr, which were dissolved into 5 mL of deionized(DI) water and the solution was mixed with a vortex mixer at roomtemperature until becoming a clear solution. 3.670 g (10 mmol) of PbBr₂was dissolved into 12.6 mL of DMF and 2.4 mL of HBr and the solution wasmixed with a magnetic stir bar at 100° C. for 10 min.

The aqueous Cs_(0.5)Rb_(0.5)Br solution was slowly added to the PbBr₂solution whilst stirring with a magnetic stir bar at room temperature(19-21° C.). At a certain point, an orange-yellow precipitate appears inthe solution. Optionally, 10 mL of ethanol was added into the solutionholding the precipitate and mixed with a vortex mixer for 10-20 seconds.The precipitate was collected on an alpha-cellulose filter paper(Advantec) and washed with ethanol a few times. Then it was dried in avacuum oven at 70° C. for 10 h.

3. Preparation of Cs_(1-x)Rb_(x)PbBr₃ Precursor Solution (ConventionalMethod)

Cs_(1-x)Rb_(x)PbBr₃ was prepared by mixing of CsPbBr₃ and RbPbBr₃precursor solutions. A CsPbBr₃ solution was prepared from CsBr andPbBr₂. An RbPbBr₃ solution was prepared from RbBr and PbBr₂. To makesaid CsPbBr₃ solution, the aforementioned halide salts (CsBr and PbBr₂)were dissolved in a solvent mixture of DMF and DMSO in 1:9 volume ratio,to obtain a perovskite solution with desired composition of 0.1 M oflead halide concentration. The solution was mixed with a magnetic stirbar at 40° C. for more than 30 min. The solution was filtered with PTFE(pore size 0.45 µm). The RbPbBr₃ solution was made by a correspondingmethod.

To produce the mixed cation perovskites Cs_(1-x)Rb_(x)PbBr₃, the CsPbBr₃and RbPbBr₃ precursor solutions were mixed to prepare aCs_(1-x)Rb_(x)PbBr₃ mixed solution where x = 0, 0.1, 0.2, 0.3, 0.4, 0.5,0.6, 0.7, 0.8 and 0.9.

4. Fabrication of CsPb(Br_(1-x)Cl_(x))₃ Crystalline Powders

CsPb(Br_(1-x)Cl_(x))₃ was prepared by mixing CsBr and CsCl (Aldrich99.9%) in various molar ratios, combined with the same molar ratio ofPbBr₂ and PbCl₂ in stoichiometric amounts. For example,CsPb(Br_(0.7)Cl_(0.3))₃ was prepared with 1.490 g (7 mmol) of CsBr and0.504 g (3 mmol) of CsCl, which were dissolved into 5 mL of DI water andthe solution was mixed with a vortex mixer until becoming clear. 2.569 g(7 mmol) of PbBr₂ and 0.834 g (3 mmol) of PbCl₂ were dissolved into 8.82mL of DMF and 1.68 mL of HBr and 3.15 mL of DMF and 1.35 mL of HCl,respectively. The PbBr₂ solution and PbCl₂ solution were mixed with amagnetic stir bar at 100° C. for 10 min and 5 min, respectively, andthen the two solutions were combined and mixed with a magnetic stir barat 18-20° C. for 5 min.

he aqueous CsBr_(0.7)Cl_(0.3) solution was slowly added into thePb(Br_(0.7)Cl_(0.3))₂ solution whilst stirring with a magnetic stir barat 18-20° C. At a certain point, a bright yellow precipitate appears inthe solution. 10 mL of ethanol was added into the solution holding theprecipitate and mixed with a vortex mixer for 10-20 seconds. Theprecipitate was taken out on an alpha-cellulose filter paper (Advantec)and washed with ethanol a few times, and then it was dried in a vacuumoven at 70° C. for 10 h.

5. Fabrication of Cs_(1-x)Rb_(x)Pb(Br_(1-x)Cl_(x))₃ Crystalline Powders

Cs_(1-x)Rb_(x)Pb(Br_(1-x)Cl_(x))₃ was prepared by mixing various molarratios of CsBr, RbBr, CsCl and RbCl combined with the same molar ratioof PbBr₂ and PbCl₂ in stoichiometric amounts. For the synthesis ofCs_(0.6)Rb_(0.4)Pb(Br_(0.7)Cl_(0.3))₃, 0.745 g (3.5 mmol) of CsBr, 0.579g (3.5 mmol) of RbBr, 0.252 g (1.5 mmol) of CsCl and 0.181 g (1.5 mmol)of RbCl were dissolved into 5 mL of D1 water and the solution was mixedwith a vortex mixer until becoming clear. 2.569 g (7 mmol) of PbBr₂ and0.834 g (3 mmol) of PbCl₂ were dissolved into 6.3 mL of DMF and 1.2 mLof HBr and 5.25 mL of DMF and 2.25 mL of HCl, respectively. The PbBr₂solution and PbCl₂ solution were mixed with a magnetic stir bar at 100°C. for 10 min and 5 min, respectively, and then the two solutions werecombined and mixed with a magnetic stir bar at 18-20° C. for 5 min.

The aqueous CsBr_(0.5)Cl_(0.5) solution was slowly added into the PbBrClsolution whilst stirring with a magnetic stir bar at 18-20° C. At acertain point, a bright yellow precipitate appears in the solution. 10mL of ethanol was added into the solution holding the precipitate andmixed with a vortex mixer for 10-20 seconds. The precipitate was takenout on an alpha-cellulose filter paper (Advantec) and washed withethanol a few times, and then it was dried in a vacuum oven at 70° C.for 10 h.

6. Fabrication of CsPb(Br_(0.7)Cl_(0.3))₃ Crystalline Powders WithExcess Cs Cation

CsPb(Br_(0.7)Cl_(0.3))₃ with excess cesium halide was prepared byvarying the molar ratio of cesium halides and lead halides.CsPb(Br_(0.7)Cl_(0.3))₃ in 2:1 molar ratio of Cs:Pb was prepared with2.8 g (14 mmol) of CsBr and 1.008 g (6 mmol) of CsCl, which weredissolved into 5 mL of DI water and mixed with a vortex mixer untilbecoming clear. 2.569 g (7 mmol) of PbBr₂ and 0.834 g (3 mmol) of PbCl₂were dissolved into 8.82 mL of DMF and 1.68 mL of HBr and 3.15 mL of DMFand 1.35 mL of HCl, respectively. The PbBr₂ solution and PbCl₂ solutionwere mixed with a magnetic stir bar at 100° C. for 10 min and 5 min,respectively, and then combined and mixed with a magnetic stir bar at18-20° C. for 5 min.

The aqueous CsBr_(0.7)Cl_(0.3) solution was slowly added into thePb(Br_(0.7)Cl_(0.3))₂ solution whilst stirring with a magnetic stir barat 18-20° C. At a certain point, a bright yellow precipitate appears inthe solution. 10 mL of ethanol was added into the solution holdingprecipitate and mixed with a vortex mixer for 10-20 seconds. Theprecipitate was taken out on an alpha-cellulose filter paper (Advantec)and washed with ethanol a few times, and then it was dried in a vacuumoven at 70° C. for 10 h.

7. Fabrication of (Cs_(1-x)Rb_(x))₄PbBr₆ Crystalline Powders

(Cs_(1-x)Rb_(x))₄PbBr₆ was prepared by varying the molar ratio of CsBrand RbBr combined with PbBr₂ in 10:1 molar ratio for Cs_(1-x)Rb_(x): Pb.For example, (Cs_(0.9)Rb_(0.1))₄PbBr₆ was prepared with 383 mg (1.8mmol) of CsBr and 33.1 mg (0.2 mmol) of RbBr, which were dissolved into600 µL of DI water and 300 µL of HBr. The salts were dissolved with avortex mixer until becoming clear. 73.4 mg (0.2 mmol) of PbBr₂ wasdissolved in 1 mL of DMSO. The PbBr₂ solution was mixed with a magneticstir bar at 100° C. for a few minutes.

The aqueous Cs_(1-x)Rb_(x)Br solution was slowly added into the PbBr₂solution whilst stirring with a magnetic stir bar at 18-20° C. At acertain point, a bright yellow precipitate appears in the solution.After addition of the entire Cs_(1-x)Rb_(x)Br solution into the PbBr₂solution, the solution was transferred to a stirring hot plate tocomplete the reaction at 100° C. for 15 minutes. 2 mL of ethanol wasthen added into the solution holding the precipitate and mixed with avortex mixer for 10-20 seconds. The precipitate was taken out on analpha-cellulose filter paper (Advantec) and washed with ethanol a fewtimes, and then it was dried in a vacuum oven at 70° C. for 10 h.

For comparative purposes, in some instances (Cs_(1-x)Rb_(x))₄PbBr₆ wasalso synthesised using an aqueous solvent, rather than an organicsolvent, as the solvent in the solution comprising the M precursor (Pb).In these experiments, the PbBr₂ was dissolved in 47 wt% HBr in water,rather than being dissolved in DMSO. Thus, (Cs_(1-x)Rb_(x))₄PbBr₆ wasprepared by varying the molar ratio of CsBr and RbBr combined with PbBr₂in 4:1 molar ratio for Cs_(1-x)Rb_(x): Pb using either HBr 47% in wateror DMSO as solvent for PbBr₂. For example, (Cs_(0.55)Rb_(0.45))₄PbBr₆was prepared with 93 mg (0.44 mmol) of CsBr and 59 mg (0.36 mmol) ofRbBr, which were dissolved into 400 µL of DI water and 50 µL of HBr. Thesalts were dissolved with a vortex mixer until becoming clear. For theHBr route, 73.4 mg (0.2 mmol) of PbBr₂ was dissolved in 1 mL of HBr. ThePbBr₂ solution was mixed with a magnetic stir bar at 20° C. for a fewminutes. For the DMSO route, 73.4 mg (0.2 mmol) of PbBr₂ was dissolvedin 1 mL of DMSO. The PbBr₂ solution was mixed with a magnetic stir barat 100° C. for a few minutes.

The aqueous Cs_(1-x)Rb_(x)Br solution was slowly added into the PbBr₂solution whilst stirring with a magnetic stir bar at 18-20° C. At acertain point, a yellow precipitate appears in the solution. 2 mL ofiso-propanol was then added into the solution holding the precipitateand mixed with a vortex mixer for 10-20 seconds. The precipitate wastaken out on an alpha-cellulose filter paper (Advantec) and washed withiso-propanol a few times, and then it was dried in a vacuum oven at 70°C. for 10 h.

Cs₄PbBr₆ and Rb₄PbBr₆ powders (where x = 0 or 1) were also prepared byproviding “A” (Cs or Rb) and “M” (Pb) in a 4:1 molar ratio. An aqueoussolution containing Cs⁺ or Rb⁺ was provided, and an organic solutioncontaining Pb²⁺. The organic solvent present in the precursor solutioncomprising Pb²⁺ was DMSO. For comparative purposes, in some cases 47 wt%HBr was used as the solvent for the precursor solution comprising Pb²⁺.

For instance, Rb₄PbBr₆ was prepared from RbBr combined with PbBr₂ in 4:1molar ratio for Rb: Pb using either HBr 47% in water or DMSO as solventfor PbBr₂. Rb₄PbBr₆ was prepared with 132 mg (0.8 mmol) of RbBr, whichwere dissolved into 400 µL of DI water and 50 µL of HBr. The salts weredissolved with a vortex mixer until becoming clear. For the HBr route,73.4 mg (0.2 mmol) of PbBr₂ was dissolved in 1 mL of HBr. The PbBr₂solution was mixed with a magnetic stir bar at 20° C. for a few minutes.For the DMSO route, 73.4 mg (0.2 mmol) of PbBr₂ was dissolved in 1 mL ofDMSO. The PbBr₂ solution was mixed with a magnetic stir bar at 100° C.for a few minutes.

The aqueous RbBr solution was slowly added into the PbBr₂ solutionwhilst stirring with a magnetic stir bar at 18-20° C. At a certainpoint, a white precipitate appears in the solution. 2 mL of iso-propanolwas then added into the solution holding the precipitate and mixed witha vortex mixer for 10-20 seconds. The precipitate was taken out on analpha-cellulose filter paper (Advantec) and washed with iso-propanol afew times, and then it was dried in a vacuum oven at 70° C. for 10 h.

8. Fabrication of (Cs_(1-x)FA_(x))₄PbI₃Br₃ Crystalline Powders

(Cs_(1-x)FA_(x))₄PbI₃Br₃ was prepared by varying the molar ratio ofCs₂CO₃ and Formamidine acetic acid (FAAC) combined with lead acetate(PbAC) in 10: 1 molar ratio for Cs_(1-x)Rb_(x): Pb. For example,(FA_(0.5)Cs_(0.5))₄PbI₃Br₃ was prepared with 81.5 mg (250 mmol) ofCs₂CO₃, 52.1 mg (500 mmol) of FAAC and 40.7 mg (125 mmol) of PbAC whichwere dissolved into 1 mL of methanol and 100 µL of acetic acid. Thesalts were mixed with a magnetic stir bar at 19-21° C. for more than 5minutes until becoming clear.

The aqueous HI/HBr (1:1 molar ratio) solution was slowly added into thesalts solution whilst stirring with a magnetic stir bar at 18-20° C. Ata certain point, a bright yellow precipitate appears in the solution andthen the color of the precipitate changed from yellow to red-orange byincreasing the amount of HI/HBr. After addition of all HI/HBr solution,3 mL of iso-propanol was immediately added into the mixed solutionholding the precipitate and mixed with a vortex mixer for 5-10 seconds.The precipitate was taken out on an alpha-cellulose filter paper(Advantec) and washed with iso-propanol a few times, and then it wasdried in a vacuum oven at 70° C. for 10 h.

9. Fabrication of Perovskite Thin Films With Stoichiometric PerovskiteMaterials

Perovskite crystals were dissolved in a mixed solvent of DMF and DMSO in1:9 volume ratio to obtain a perovskite solution with desiredcomposition and molar concentration of 0.125-0.3 M. The perovskiteprecursor solution was coated onto the glass substrate by spin-coatingat 3000 rpm (acceleration 500 rpm) for 50 sec in a normal lab atmosphere(18-20° C., humidity 40-60 %), and then the perovskite films were driedat 100° C. for 5 min in the same atmosphere.

10. Fabrication of Thin Films of Polycrystalline Perovskites Dispersedin a Polymer Matrix, With Excess a Site Perovskite

The crystals were grinded in a mortar for 5 minutes and added into apoly(methyl methacrylate) solution (PMMA, 200 mg/mL in toluene) toobtain a perovskite crystal dispersion. The mixed perovskite/polymersolution was coated on glass substrate using the doctor blade method.The film was dried on a hot plate at 70° C. for a few minutes.

11. Characterization

UV-vis absorption spectra were measured by a commercialspectrophotometer (Varian Cary 300 UV-Vis). Photoluminescence (PL)spectra were recorded using an excitation wavelength of 365-405 nm andslit widths of 1-3 mm on a commercial spectrofluorometer (Horiba,Fluorolog). Photoluminescence quantum yield (PLQY) values weredetermined using a 405 nm CW laser (RLTMDL-405, Roithner LasertechnikGmbH) to illuminate a sample in an integrating sphere (Oriel Instruments70682NS), and the laser scattered and PL collected using a fiber-coupleddetector (Ocean Optics MayaPro). PLQY calculations were carried outusing established techniques. The laser intensity was adjusted usingneutral density filters. Powder X-ray reflection diffraction (PXRD)patterns were obtained with a Panalytical X′pert powder diffractometer(Cu-Kα1 radiation; λ = 154.05 pm) at room temperature.

12. Results and Discussion

Thin films of Cs_(1-x)Rb_(x)PbBr₃ with x = 0 to x = 1 were fabricated asin sections (2) and (9). This type of mixed cation lead halideperovskites is of interest for blue and green light emitters. In FIGS. 1(a) and (b), X-ray diffraction (XRD) patterns of Cs_(1-x)Rb_(x)PbBr₃ forsynthesized powders and thin films, respectively, are shown. It wasobserved that the synthesized Cs_(1-x)Rb_(x)PbBr₃ powders up to x=0.3show similar peak positions than neat CsPbBr₃ in the orthorhombicstructure (space group: pnma). When further increasing Rb concentrationin Cs_(1-x)Rb_(x)PbBr₃, a diffraction peak at ~11.8 degree appears,which is related to non-perovskite NH₄CdCl₃ like structure. However, inCs_(1-x)Rb_(x)PbBr₃ thin films, a highly ordered perovskite phase up tox= 0.9 was observed. The peak positions at 15.2° and 30.7° on CsPbBr₃composition shift to higher angle with increasing Rb concentration asexpected from the reduction of the lattice size due to the smaller ionicradius of Rb with respect to Cs.

This illustrates the improvement in stoichiometric control (up to x =0.9) that can be obtained by fabricating thin films according the methodof the invention.

Similarly, FIGS. 2 (a) and 2 (b)show UV-Vis absorption spectra ofCs_(1-x)Rb_(x)PbBr₃ for synthesized powders and thin films,respectively. For powders, the absorption onset of Cs₁₋ _(x)Rb_(x) PbBr₃shows a slight shift in onset towards 550 nm as x varies from 0 (filledsquares) to 0.6 (right-facing triangle). This illustrates the change inoptical properties achieved by fabricating different products accordingto the method of the invention. However, even more marked shifts areshown in the data for thin films in FIG. 2(b): the absorption onset ofCs₁₋ _(x)Rb_(x)PbBr₃ demonstrates a significant shift towards the blueas x increases from 0. The shift in absorption wavelength is visiblewhen x is as low as 0.3. The observation of a steady shift in absorptiononset is consistent with the formation of a stoichiometric mixture ofcations in the perovskite lead bromide structure. The steady shift alsoillustrates that materials with finely tunable optical properties areobtainable by the method of the invention.

FIGS. 3 (a) and (b)show steady-state photoluminescence (SSPL) spectraand normalized photoluminescence (PL) spectra, respectively, ofCs_(1-x)Rb_(x)PbBr₃ for synthesized crystalline powders. The PLintensity shows a considerable increase where x is in the range 0.2 to0.7, and the normalised PL emission peak clearly moves (albeit without aclear trend with increasing Rb content), similar to the absorptionspectra in FIG. 2 (a). In FIGS. 3 (c) and (d), SSPL spectra andnormalized PL spectra, respectively, of Cs_(1-x)Rb_(x)PbBr₃ thin filmsare shown. The PL intensity of the thin films is significantly enhancedwith the incorporation of Rb into neat CsPbBr₃ and the largest PLintensity is reached with x= 0.6 (FIG. 3 (c)). Photoluminescence quantumyields (PLQY) for the thin films for x= 0 and x = 0.6 are found to be3.1% and 18.3%, respectively. In FIG. 3 (d), the PL peak position ofCs_(1-x)Rb_(x)PbBr₃ does not demonstrate a significant shift up to x=0.3, in agreement with the shift of the absorption edge shown in FIG. 2(b). With further increase of Rb concentration, a clear shift towardsshorter wavelengths is observed.

The structural and optical properties of mixed Cs_(1-x)Rb_(x)PbBr₃ thinfilms shown in FIGS. 1-3 demonstrate that the invented method isbeneficial for the synthesis of ordered and homogeneously mixed leadbromide perovskites. In order to further assess the advantages of themethod of the invention, Cs_(1-x)Rb_(x)PbBr₃ thin films were preparedusing a conventional method. The optical properties of the thin filmsobtained from both methods were compared (see FIG. 4 ).

It was observed that for x= 0 and x = 0.7 (FIG. 4 (a)), the absorptiononset of films cast from the conventional method (i.e. dissolvedCsBr/RbBr and PbBr₂ in DMF:DMSO) is located at a lower wavelengthcompared to the films obtained with the method of the invention(“invented method”, i.e. formation of thin films from powders fabricatedfrom two solutions and redissolved in DMSO to form a thin film). Forhigh Rb content (x = 0.7), films obtained from the conventional methodshow a large shift between the absorption onset and the PL peak,centered at ~525 nm. In contrast, the PL peak position of the x= 0.7thin film fabricated with the invented method is at ~ 500 nm with almostno Stokes shift. Additionally, the PL intensity of the thin filmsobtained with the invented method is much higher than that of the thinfilm obtained with the conventional method. The large shift between theabsorption onset and the PL peak in the films obtained with theconventional method is due to inhomogeneous mixing of the cations in theperovskite structure. PL emission at 525 nm, indicates the presence ofnearly neat cesium lead bromide regions, indicative of poor mixing. Incontrast, the PL emission of the films obtained with the method of theinvention is consistent with a single phase, homogeneous material.

It is also demonstrated herein that the method of the invention isparticularly beneficial for the fabrication ofstoichiometrically-homogeneous mixed halide and mixed cationperovskites. In FIGS. 5 and 6 , an increase of the semiconductor bandgapis shown with addition of chloride and rubidium, respectively. Adecrease in the PL intensity was observed with addition of chloride ionthe bromide-based perovskite. The PLQY of the films with x= 0 and x =0.3 is determined to be 3-4 % and < 1 %, respectively.

FIG. 6 shows that the PL intensity increases with the addition ofrubidium ions for a given Br/Cl ratio (CsPb(Br_(0.7)Cl_(0.3))₃). ThePLQY of the films wth x= 0 and x = 0.3 is determined to be < 1% and1-2%, respectively.

In another example, the method of the invention was used to generatehighly emissive lead bromide perovskites with an excess of A cation(that is, a perovskite wherein the Cs:Pb ratio is greater than 1). FIG.7 shows the effect of increasing the molar ratio of Cs:Pb in aCsPb(Br_(0.7)Cl_(0.3))₃ composition. A significant enhancement of the PLintensity with increasing Cs content is observed. The PL peak positionvaries slightly, is attributed to optical effects (e.g. reabsorption)rather than changes of the emitter bandgap. The PLQY of 2:1 compositionis determined to be 6.6 ± 0.8%. The excitonic absorption peak in thefilm with 1:1 disappears with increasing Cs content (FIG. 7 (c)). PowderX-ray diffraction (PXRD) data shown in FIG. 7 (d)that the 2:1 thin filmshows different orientation on the thin film as compared with 1:1composition.

From these experiments, it is concluded that excess Cs (i.e. “A” site inperovskite structure) changes positively the optical properties but alsomodifies the crystallographic properties. FIG. 8 shows PXRD ofCs_(x)PbBr_(y) crystals with different Cs:Pb molar ratios in thestarting solutions, where x and y depend on CsBr content. With highCs:Pb ratio (8:1), we clearly observe the presence of a uniquediffraction peak at 2 theta= 12.7° in the PXRD pattern, corresponding toa low-dimensional perovskite (zero dimensional or 0D structure) with aK₄CdCl₆ -like structure [Saidaminov et al., ACS Energy Letters, 2016,1(4), pp.840-845]. FIG. 8 (b), shows that an excess Cs content increasesthe light absorption around 310 nm, which relates to a formation of 0-DCs₄PbBr₆ [Yang et al., Chem. Mater. 2017, 29, pp 8978 -8982], althoughthe absorption onset regarding to three dimensional (3-D) CsPbBr₃structure remains. Meanwhile, the excess Cs content enhancesdramatically the photoluminescence intensity at 520 nm and the PLQY ofthe powders of 8:1 and 10:1 Cs:Pb ratio are determined to be 19.5 ± 1.18% and 24.6 ± 1.21 %, respectively (FIG. 8 (c)). These results indicatethat the excess Cs composition contains small 3-D CsPbBr₃ regions in the0-D Cs₄PbBr₆ matrix [Quan et al., Adv. Mater. 2017, 29, 1605945].

The method of the invention was used to synthesize highly emissiveperovskite crystalline powders with generic composition(Cs_(1-x)Rb_(x))₄PbBr₆. FIG. 9 shows PXRD of (Cs₁. _(x)Rb_(x))₄PbBr₆with low Rb content (x=0.05 to x=0.2). With incorporation of Rb into theCs₄PbBr₆ crystals, the XRD pattern is seen to move to a higher 2 thetaangle due to the smaller ionic radius of Rb as compared to Cs. Thesepowders are were dispersed in a thin film (as described in 10) and theiroptical properties examined. FIG. 10 shows the PL emission spectra of(Cs_(1-x)Rb_(x))₄PbBr₆ powders dispersed in a PMMA matrix. An increasein the PL intensity is seen with increasing Rb content, but nosignificant shift of the PL peak position. The PLQY of the dispersedfilms with x= 0, 0.05, 0.1, and 0.2 was determined to be 78.0 + 4.19 %,77.6 + 4.88 %, 92.3 + 4.65 % and 94.3 + 4.88 % respectively. Thus, thepresence of Rb significantly increases the PLQY of the dispersed films.

Although no significant shift of the photoluminescence peak is seen inthe initial photoluminescence spectrum, a small shift in thephotoluminescence peak position can be observed with varying Rb contentin the steady state photoluminescence spectrum. FIG. 16 , graph (a)shows the normalised steady-state photoluminescence spectra of (Cs₁₋_(x)Rb_(x))₄PbBr₆ powders with different Rb content. As x increases from0 to 0.5, a slight blue shift is observed.

The photostability and PLQY of the (Cs_(1-x)Rb_(x))₄PbBr₆ powders wasalso tested. FIG. 16 , graph (b) shows the PLQY of(Cs_(1-x)Rb_(x))₄PbBr₆ powders, as function of exposure time undersimulated sun light (76 mW·cm⁻² at 60° C.) for exposures of up to 600hours. It is clear from this figure that the inclusion of Rb enhancesdramatically the PLQY of the powders produced. Where x =0.1, the initialPLQY increases from around 30% (where x = 0) to around 40%. Further,where x = 0.4 or 0.5, the PLQY increases to ~65 %, No drastic differencein photostability is observed for these powders, as can be seen fromtheir similar decay over 600 hours. However, it is clear that theenhanced PLQY of the rubidium-containing powders is stable for veryextended periods. For instance, after 500 hours, the PLQY of the x = 0.4and x = 0.5 powders appears to still be over 30%; better than any PLQYobserved in the absence of rubidium.

The effect of varying the ratio of “A” to “M” in the(Cs_(1-x)Rb_(x))₄PbBr₆ powders was also tested. FIG. 17 , graph (a),shows the normalised steady-state photoluminescence spectra of(Cs_(0.55)Rb_(0.45))₄PbBr₆ powders as the molar ratio between [A] (thatis, Cs⁺ and Rb⁺) and [M] (that is, Pb²⁺) varies. It is observed that thepresence of excess (Cs_(0.55)Rb_(0.45)) (which occupies the “A” site inperovskite structure) shifts the photoluminescence peak position to theblue, i.e. to higher energy wavelengths.

The photostability under irradiation of these structures was alsotested. FIG. 17 , graph (b) shows the PLQY of (Cs_(0.55)Rb_(0.45))₄PbBr₆powders having differing molar ratio between [A] and [M], as function ofexposure time under simulated sun light (76 mW·cm⁻² at 60° C.) forexposures of up to 140 hours. The powder with an [A]:[M] ratio of 10:1shows the highest PLQY (~70%). This is exceptionally high. Moreover, thepowders of formula (Cs_(0.55)Rb_(0.45))₄PbBr₆ all have very goodstability under exposure to simulated sunlight. All tested powders havea PLQY exceeding about 40% even after 120 hours of exposure to simulatedsunlight. The powders did not show a significant difference inphotostability as the [A]: [M] ratio varied.

In another example, the method of the invention was used to preparemixed iodide/bromide lead perovskites. These are promisingsemiconducting materials for both light-emitting and photovoltaicdevices. (Cs_(1-x)FA_(x))₄PbI₃Br₃ compositions (where FA isformamidinium CH₂(NH₂)₂ ⁺) were fabricated in this instance. Thesematerials present a bandgap in the red region of the visible spectrum.FIG. 11 shows the PXRD patterns of (Cs _(1-x)FA_(x))₄PbI₃Br₃. Similar toCs₄PbBr₆, it was observed that the iodide-containing compositions alsoform vacancy-ordered perovskites with a K₄CdCl₆-like structure. Onincorporation of FA into Cs₄PbI₃Br₃, a shift in the PXRD pattern shifttowards lower angles was observed, which is consistent with theincorporation of the larger ionic radius cation (i.e. FA) into thelattice to form an homogeneous material.

FIG. 12 shows the PL emission spectra of (Cs_(1-x)FA_(x))₄PbI₃Br₃crystalline powders dispersed in a PMMA matrix. Incorporating FAincreases the PL intensity and induces is a red shift in the PL peakposition consistent with the decrease of the bandgap related to theincrease of the crystalline lattice.

The method of the invention can produce crystalline A/M/X material offormula [A]_(a)[M]_(b)[X]_(c), with excellent control over thestoichiometry of the product. This method therefore has the advantagesof producing reduced side-products, and hence a purer product. Toillustrate this, various products of formula [A]₄[M][X]₆ were producedby the method of the invention, and also by a corresponding methodwherein the M precursor was provided in an aqueous solvent rather thanan organic solvent. The resulting products were analysed and compared.

FIG. 18 (graph (a)) shows steady-state photoluminescence spectra ofCs₄PbBr₆ powders obtained via the method of the invention and by analternative method as mentioned above. FIG. 18 (graph (b)) shows thesame spectra, normalised. It is clear from FIG. 18(a) that the method ofthe invention leads to a product with a much higher photoluminescenceintensity. The normalised spectrum of Cs₄PbBr₆ powders synthesised bythe alternative method shows that the spectrum of the product producedby the alternative method in fact presents a split peak. One maximum isseen at 510 nm and another at 540 nm. By contrast, Cs₄PbBr₆ powderssynthesised by the method of the invention show a single peak. It isclear therefore that the method of the invention permits finer controlover the stoichiometry of the product, and hence a product with improvedoptical properties.

The products of the differing synthesis were also analysed by X-raydiffraction, as shown in FIG. 19 . The uppermost trace shows the X-raydiffraction pattern of Cs₄PbBr₆ powders synthesised by the alternativemethod, wherein the M precursor was initially provided in aqueoushydrobromic acid. The middle trace shows the X-ray diffraction of theproduct produced by the method of the invention, wherein the M precursorwas provided in an organic solvent, DMSO. The bottom trace is thecalculated trace for Cs₄PbBr₆ in space group R-3cH. In each trace,circles represent an X-Ray diffraction peak position of CsPbBr₃ inorthorhombic structure while squares represent an X-ray diffraction peakposition of CsBr.

Both synthetic routes generate Cs₄PbBr₆. However, powders synthesised bythe method of the invention contain mainly Cs₄PbBr₆, but there is asmall peak, which can be assigned to CsPbBr₃. On the other hand,Cs₄PbBr₆ powders synthesised using a hydrobromic acid solutioncontaining lead additionally contain X-ray diffraction peaks which canbe assigned to CsBr and CsPbBr₃. In particular, the peak intensity andnumber of peaks related to CsPbBr₃ observed in the powder productsynthesised by the alternative method increase drastically in comparisonwith that synthesised by the method of the invention.

Powders of formula [A]₄[M][X]₆ containing both Rb and Cs A species werealso synthesised by the method of the invention and by an alternativemethod using a hydrobromic acid solvent in place of DMSO. Thesteady-state photoluminescence spectra of the products,(Cs_(0.55)Rb_(0.45))₄PbBr₆ powders, are shown in FIG. 20 . The spectrain FIG. 20(b) are normalised; those in FIG. 20(a) are not. It is clearfrom FIG. 20(a) that the method of the invention leads to a substantialincrease in the photoluminescence intensity of the product, compared tothe product produced by the alternative method. The normalisedsteady-state photoluminescence spectra of (Cs_(0.55)Rb_(0.45))₄PbBr6powders synthesised by the alternative method presents a split peak withmaxima at 510 nm and 540 nm, whereas Cs₄PbBr₆ powders synthesised by themethod of the invention present a single peak at 515 nm.

The X-ray diffraction patterns of (Cs_(0.55)Rb_(0.45))₄PbBr6 powderssynthesised using the method of the invention, and an alternativemethod, were also obtained. These are shown in FIG. 21 . The powderproduced by the method of the invention appears in the middle spectrum,and the powder produced by the alternative method using an aqueoushydrobromic acid solvent for the lead species is shown in the topspectrum. Also marked, in the bottom spectrum, is the X-ray diffractionpattern calculated for Cs₄PbBr₆ (bottom) in space group R-3cH. Alsomarked are the X-ray diffraction peak positions of CsPbBr₃ inorthorhombic structure, marked with filled-in circles.

It is clear from comparing the two experimental spectra in FIG. 21 thatboth synthetic routes generate (Cs_(0.55)Rb_(0.45))₄PbBr₆.(Cs_(0.55)Rb_(0.45))₄PbBr6 powders synthesised by the method of theinvention (with DMSO as the solvent of the M precursor) shows thediffraction pattern attributed to Cs₄PbBr₆ only, indicating that theproduct is produced with very few side-reactions and side-products. Onthe other hand, (Cs_(0.55)Rb_(0.45))₄PbBr₆ powders synthesised by thealternative method contain diffraction peaks which can be assigned asCsPbBr₃. Notably, the diffraction peak for the “excess” A site (lowdimensional perovskites) is absent on the powders synthesised by thealternative method.

Powders of formula Rb₄PbBr₆ containing were also synthesised by themethod of the invention and by an alternative method using a hydrobromicacid solvent in place of DMSO. The X-ray diffraction patterns of powderssynthesised by each method are shown in FIG. 22 . The X-ray diffractionpattern of the powder produced by the method of the invention can beattributed to Rb₄PbBr₆. The X-ray diffraction pattern of the product ofthe alternative method could not be fitted. However, it differssignificantly from the Rb₄PbBr₆ pattern observed in the product of themethod of the invention.

The inventors have appreciated that the stoichiometry of the A/M/Xmaterial produced by the method of the invention may depend not only onthe molar ratio of [A], [M] and [X] initially supplied but may also beinfluenced by the choice of solvent in the [M] precursor. This isillustrated by FIGS. 19 and 21 , which show that the solvent used in themethod of the invention may have an effect on the specific A/M/Xcrystalline material produced. For instance, it is clear from the abovethat [A][M][X]₃ readily forms when DMF is used as the organic solventfor the M precursor in the formation of lead halide perovskitescontaining Rb and/or Cs as [A]. However, where DMSO is used, and theappropriate molar ratio of [A]:[M] is provided, the method of theinvention favours the [A]₄[M][X]₆ structure. That is clear from the“DMSO” traces in FIGS. 19 and 21 , which show that little [A][M][X]₃ ispresent and that [A]₄[M][X]₆ is the major product. Thus, thestoichiometry of the A/M/X material produced by the method of theinvention may be tuned by the choice of solvent. The skilled person isreadily able to select different solvents for the purpose of tuningstoichiometry, and producing particular desired A/M/X materials, inlight of the information presented herein.

1-29. (canceled)
 30. A process for producing a crystalline A/M/Xmaterial, which crystalline A/M/X material comprises a compound offormula:

wherein: [A] comprises one or more A cations; [M] comprises one or moreM cations which are metal or metalloid cations; [X] comprises one ormore X anions; a is a number from 1 to 6; b is a number from 1 to 6; andc is a number from 1 to 18, the process comprising: a) contacting anaqueous solution comprising an A precursor and an aqueous solvent withan organic solution comprising an M precursor and an organic solvent;and b) allowing a precipitate to form when the said aqueous and organicsolutions are contacted, and wherein the process further comprisesproducing an optoelectronic device comprising the crystalline A/M/Xmaterial, wherein the optoelectronic device is a photovoltaic device.31. A process according to claim 30 which is a process for preparing athin film of said crystalline A/M/X material, the process furthercomprising: a) optionally washing the precipitate; b) dissolving theprecipitate in an organic solvent to form a film-forming solution; andc) dispersing the film-forming solution on a substrate, wherein saidoptoelectronic device comprises the thin film of the crystalline A/M/Xmaterial.
 32. A process according to claim 30 which is a process forpreparing a thin film of said crystalline A/M/X material, the processfurther comprising: c′) optionally washing the precipitate; d′)vapourising the precipitate; and e′) depositing the vapourisedprecipitate on a substrate, wherein said optoelectronic device is anoptoelectronic device comprising the thin film of said crystalline A/M/Xmaterial.
 33. A process according to claim 30 wherein said one or more Acations are monocations and said one or more M cations are dications.34. A process according to claim 30 wherein the compound of formula[A]_(a[)M]_(b[)X]_(c) is a compound of formula [A][M][X]₃.
 35. A processaccording to claim 30 wherein the compound of formula[A]_(a[)M]_(b[)X]_(c) is a compound of formula [A]₂[M][X]₆.
 36. Aprocess according to claim 30 wherein the crystalline A/M/X materialcomprises two or more compounds of formula [A]_(a)[M]_(b)[X]_(c).
 37. Aprocess according to claim 30 wherein [A] comprises two or moredifferent A cations.
 38. A process according to claim 37 wherein theprocess comprises, prior to step (a), preparing the aqueous solution bycombining a solution of a first A precursor with a solution of a secondA precursor, wherein: the first A precursor comprises a first A cation;and the second A precursor comprises a second A cation.
 39. A processaccording to claim 30 wherein [X] comprises two or more different Xanions.
 40. A process according to claim 39 wherein the processcomprises, prior to step (a), preparing the organic solution bycombining a solution of a first M precursor with a solution of a secondM precursor, wherein: the first M precursor comprises a first X anion;and the second M precursor comprises a second X anion.
 41. A processaccording to claim 30 wherein the organic solution comprises ahydrohalic acid.
 42. A process according to claim 30 wherein the organicsolution comprises a hydrohalic acid of formula HX′, wherein X′ is oneof the X anions.
 43. A process according to claim 30 wherein therelative molar amounts of each of said one or more A cations, said oneor more M cations and said one or more X anions in the aqueous solutiontogether with the organic solution are equal to the relative molaramounts of each of said one or more A cations, said one or more Mcations and said one or more X anions in the compound of formula[A]_(a)[M]_(b)[X]_(c).
 44. A process according to claim 30 wherein eachA cation is selected from an alkali metal cation, C₁₋₁₀ alkylamammonium,C₂₋₁₀ alkenylammonium, C₁₋₁₀ alkyliminium, C₃₋₁₀ cycloalkylamammoniumand C₃₋₁₀ cycloalkyliminium optionally substituted with one or moresubstituents selected from amine, C₁₋₆ alkylamine, imine, C₁₋₆alkylimine, C₁₋₆ alkyl, C₂₋₆ alkenyl,, C₃₋₆ cycloalkyl and C₆₋₁₂ aryl;preferably one or more of Cs⁺, Rb⁺, methylammonium, ethylammonium,propylammonium, butylammonium, pentylammoium, hexylammonium,septylammonium, octylammonium, and guanidinium.
 45. A process accordingto claim 30 wherein each A precursor is a halide salt of the A cation orone of the A cations.
 46. A process according to claim 30 wherein eachmetal or metalloid M cation is selected from Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺,Ni²⁺, Mn²⁺, Fe²⁺, Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺,preferably Sn²⁺, Pb²⁺, Cu²⁺, Ge²⁺, and Ni²⁺; particularly preferablyPb²⁺.
 47. A process according to claim 30 wherein each M precursor is ahalide salt of the M cation or one of the M cations.
 48. A processaccording to claim 30 wherein each X anion is selected from F⁻, Cl⁻, Br-or I⁻, preferably Cl⁻ or Br⁻.
 49. A process according to claim 30wherein [A]_(a[)M]_(b[)X]_(c) comprises at least two different A cationsand at least two different X anions.
 50. A process according to claim 30wherein [A] comprises Cs⁺ and Rb⁺ and wherein [X] comprises Br⁻ and Cl⁻.51. A process according to claim 30 wherein said aqueous solventcomprises less than 20% by volume of organic solvents, preferably lessthan 10% by volume of organic solvents.
 52. A process according to claim30 wherein the organic solvent in the organic solution comprises lessthan 20% by volume of water, preferably less than 10% by volume ofwater.
 53. A process according to claim 30 wherein the organic solventin the organic solution comprises a polar organic solvent, preferably anorganic solvent that is miscible with water, further preferably DMF. 54.A process according to claim 30 wherein the organic solvent of step (d)comprises a polar solvent, preferably DMSO and/or DMF.
 55. A processaccording to claim 30 wherein step (e) comprises spin-coating thefilm-forming solution on the substrate.
 56. A process according to claim30 wherein the process further comprises: d) removing the organicsolvent from the film-forming solution on the substrate, preferably byevaporation, preferably at a temperature of 15 to 150° C., alsopreferably for at least 1 minute.
 57. A process according to claim 30wherein the photovoltaic device is a solar cell.
 58. An optoelectronicdevice which is obtainable by or obtained by a process as defined inclaim 30, wherein the optoelectronic device is a photovoltaic device.59. An optoelectronic device according to claim 58 wherein thephotovoltaic device is a solar cell.